Network Working Group P. Saint-Andre
Internet-Draft Cisco
Obsoletes: 3920 (if approved) March 8, 2010
Intended status: Standards Track
Expires: September 9, 2010
Extensible Messaging and Presence Protocol (XMPP): Coredraft-ietf-xmpp-3920bis-05
Abstract
This document defines the core features of the Extensible Messaging
and Presence Protocol (XMPP), a technology for streaming Extensible
Markup Language (XML) elements for the purpose of exchanging
structured information in close to real time between any two or more
network-aware entities. XMPP provides a generalized, extensible
framework for incrementally exchanging XML data, upon which a variety
of applications can be built. The framework includes methods for
stream setup and teardown, channel encryption, authentication of a
client to a server and of one server to another server, and
primitives for push-style messages, publication of network
availability information ("presence"), and request-response
interactions. This document also specifies the format for XMPP
addresses, which are fully internationalizable.
This document obsoletes RFC 3920.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 9, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Internet-Draft XMPP Core March 20101. Introduction1.1. Overview
The Extensible Messaging and Presence Protocol (XMPP) is an
application profile of the Extensible Markup Language [XML] for
streaming XML data in close to real time between any two (or more)
network-aware entities. XMPP is typically used to exchange messages,
share presence information, and engage in structured request-response
interactions. The basic syntax and semantics of XMPP were developed
originally within the Jabber open-source community, mainly in 1999.
In late 2002, the XMPP Working Group was chartered with developing an
adaptation of the core Jabber protocol that would be suitable as an
IETF instant messaging (IM) and presence technology. As a result of
work by the XMPP WG, [RFC3920] and [RFC3921] were published in
October 2004, representing the most complete definition of XMPP at
that time.
As a result of extensive implementation and deployment experience
with XMPP since 2004, as well as more formal interoperability testing
carried out under the auspices of the XMPP Standards Foundation
(XSF), this document reflects consensus from the XMPP developer
community regarding XMPP's core XML streaming technology. In
particular, this document incorporates the following backward-
compatible changes from RFC 3920:
o Incorporated corrections and errata
o Added examples throughout
o Clarified and more completely specified matters that were
underspecified
o Modified text to reflect updated technologies for which XMPP is a
using protocol, e.g., Transport Layer Security (TLS) and the
Simple Authentication and Security Layer (SASL)
o Defined several additional stream, stanza, and SASL error
conditions
o Removed the deprecated DIGEST-MD5 SASL mechanism [DIGEST-MD5] as a
mandatory-to-implement technology
o Added the TLS plus the SASL PLAIN mechanism [PLAIN] as a
mandatory-to-implement technology
o Defined of optional support for multiple resources over the same
connection
o Transferred historical documentation for the server dialback
protocol from this specification to a separate specification
Therefore, this document defines the core features of XMPP 1.0, thus
obsoleting RFC 3920.
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Note: [XMPP-IM] defines the XMPP features needed to provide the
basic instant messaging and presence functionality that is
described in [IMP-REQS].
1.2. Functional Summary
This non-normative section provides a developer-friendly, functional
summary of XMPP; refer to the sections that follow for a normative
definition of XMPP.
The purpose of XMPP is to enable the exchange of relatively small
pieces of structured data (called "XML stanzas") over a network
between any two (or more) entities. XMPP is implemented using a
client-server architecture, wherein a client needs to connect to a
server in order to gain access to the network and thus be allowed to
exchange XML stanzas with other entities (which can be associated
with other servers). The process whereby a client connects to a
server, exchanges XML stanzas, and ends the connection is:
1. Determine the hostname and port at which to connect
2. Open a TCP connection
3. Open an XML stream
4. Complete TLS negotiation for channel encryption (recommended)
5. Complete SASL negotiation for authentication
6. Bind a resource to the stream
7. Exchange an unbounded number of XML stanzas with other entities
on the network
8. Close the XML stream
9. Close the TCP connection
Within XMPP, one server can optionally connect to another server to
enable inter-domain or inter-server communication. For this to
happen, the two servers need to negotiate a connection between
themselves and then exchange XML stanzas; the process for doing so
is:
1. Determine the hostname and port at which to connect
2. Open a TCP connection
3. Open an XML stream
4. Complete TLS negotiation for channel encryption (recommended)
5. Complete SASL negotiation for authentication *
6. Exchange an unbounded number of XML stanzas both directly for the
servers and indirectly on behalf of entities associated with each
server (e.g., connected clients)
7. Close the XML stream
8. Close the TCP connection
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* Note: Depending on local service policies, it is possible that a
deployed server will use the older server dialback protocol to
provide weak identity verification in cases where SASL negotiation
would not result in strong authentication (e.g., because TLS
negotiation was not mandated by the peer server, or because the
certificate presented by the peer server during TLS negotiation is
self-signed and thus provides only weak identity); for details,
see [XEP-0220].
In the sections following discussion of XMPP architecture and XMPP
addresses, this document specifies how clients connect to servers and
specifies the basic semantics of XML stanzas. However, this document
does not define the "payloads" of the XML stanzas that might be
exchanged once a connection is successfully established; instead,
those payloads are defined by various XMPP extensions. For example,
[XMPP-IM] defines extensions for basic instant messaging and presence
functionality. In addition, various specifications produced in the
XSF's XEP series [XEP-0001] define extensions for a wide range of
more advanced functionality.
1.3. Conventions
The following capitalized keywords are to be interpreted as described
in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
"SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
"OPTIONAL".
Certain security-related terms are to be understood in the sense
defined in [SECTERMS]; such terms include, but are not limited to,
"assurance", "attack", "authentication", "authorization",
"certificate", "certification authority", "confidentiality",
"credential", "downgrade", "encryption", "fingerprint", "hash value",
"identity", "integrity", "signature", "security perimeter", "self-
signed certificate", "sign", "spoof", "tamper", "trust", "trust
anchor", "trust chain", "validate", "verify". Other security-related
terms (for example, "denial of service") are to be understood in the
sense defined in the referenced specifications.
The term "whitespace" is used to refer to any character that matches
production [3] content of [XML], i.e., any instance of SP, HT, CR,
and LF.
Following the "XML Notation" used in [IRI] to represent characters
that cannot be rendered in ASCII-only documents, some examples in
this document use the form "&#x...." as a notational device to
represent Unicode characters (e.g., the string "&#x0159;" stands for
the Unicode character LATIN SMALL LETTER R WITH CARON).
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In examples, lines have been wrapped for improved readability,
"[...]" means elision, and the following prepended strings are used
(these prepended strings are not to be sent over the wire):
o C: = a client
o E: = any XMPP entity
o I: = an initiating entity
o P: = a peer server
o R: = a receiving entity
o S: = a server
o S1: = server1
o S2: = server2
1.4. Acknowledgements
The editor of this document finds it impossible to appropriately
acknowledge the many individuals who have provided comments regarding
the protocols defined herein. However, thanks are due to those who
have who have provided implementation feedback, bug reports, requests
for clarification, and suggestions for improvement since the
publication of the RFC this document supersedes. The editor has
endeavored to address all such feedback, but is solely responsible
for any remaining errors and ambiguities.
1.5. Discussion Venue
[[ RFC Editor: please remove this section. ]]
The document editor and the broader XMPP developer community welcome
discussion and comments related to the topics presented in this
document. The primary and preferred venue is the <xmpp@ietf.org>
mailing list, for which archives and subscription information are
available at <https://www.ietf.org/mailman/listinfo/xmpp>. Related
discussions often occur on the <standards@xmpp.org> mailing list, for
which archives and subscription information are available at
<http://mail.jabber.org/mailman/listinfo/standards>.
2. Architecture
XMPP provides a technology for the asynchronous, end-to-end exchange
of structured data by means of direct, persistent XML streams among a
distributed network of globally-addressable, presence-aware clients
and servers. Because this architectural style involves ubiquitous
knowledge of network availability and a conceptually unlimited number
of concurrent information transactions in the context of a given
client-to-server or server-to-server session, we label it
"Availability for Concurrent Transactions" (ACT) to distinguish it
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from the "Representational State Transfer" [REST] architectural style
familiar from the World Wide Web. Although the architecture of XMPP
is similar in important ways to that of email (see [EMAIL-ARCH]), it
introduces several modifications to facilitate communication in close
to real time. The salient features of this ACTive architectural
style are as follows.
2.1. Global Addresses
As with email, XMPP uses globally-unique addresses (based on the
Domain Name System) in order to route and deliver messages over the
network. All XMPP entities are addressable on the network, most
particularly clients and servers but also various additional services
that can be accessed by clients and servers. In general, server
addresses are of the form "domain.tld" (e.g., "im.example.com"),
accounts hosted at a server are of the form "localpart@domain.tld"
(e.g., "juliet@im.example.com"), and a particular connected device or
resource that is currently authorized for interaction on behalf of an
account is of the form "localpart@domain.tld/resource" (e.g.,
"juliet@im.example.com/balcony"). XMPP addresses are defined under
Section 3.
2.2. Presence
XMPP includes the ability for an entity to advertise its network
availability or "presence" to other entities. Such availability for
communication is signalled end-to-end via dedicated communication
primitives in XMPP (the <presence/> stanza). Although knowledge of
network availability is not strictly necessary for the exchange of
XMPP messages, it facilitates real-time interaction because the
originator of a message can know before initiating communication that
the intended recipient is online and available. XMPP presence is
defined in [XMPP-IM].
2.3. Persistent Streams
Availability for communication is also built into point-to-point
connections (e.g., a discrete client-to-server or server-to-server
connection) through the use of direct, persistent XML streams between
the entity that initiated the connection (either a client or a
server) and the entity that received the connection (a server). Thus
either party to a stream knows that it can immediately push data to
the other party for immediate routing or delivery. XML streams are
defined under Section 5.
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The basic unit of meaning in XMPP is not an XML stream (which simply
provides the transport for point-to-point communication) but an XML
"stanza", which is essentially a fragment of XML that is sent over a
stream. The root element of a stanza includes routing attributes
(such as "from" and "to" addresses) and the child elements of the
stanza contain a payload for delivery to the intended recipient. XML
stanzas are defined under Section 9.
2.5. Distributed Network
In practice, XMPP consists of a network of clients and servers that
inter-communicate (however, communication between any two given
deployed servers is strictly OPTIONAL). Thus, for example, the user
<juliet@im.example.com> associated with the server <im.example.com>
might be able to exchange messages, presence, and other structured
data with the user <romeo@example.net> associated with the server
<example.net>. This pattern is familiar from messaging protocols
that make use of global addresses, such as the email network (see
[SMTP] and [EMAIL-ARCH]). As a result, end-to-end communication in
XMPP is logically peer-to-peer but physically client-to-server-to-
server-to-client, as illustrated in the following diagram.
example.net ---------------- im.example.com
| |
| |
romeo@example.net juliet@im.example.com
Note: Architectures that employ XML streams (Section 5) and XML
stanzas (Section 9) but that establish peer-to-peer connections
directly between clients using technologies based on [LINKLOCAL]
have been deployed, but such architectures are not described in
this specification and are best described as "XMPP-like"; for
details, see [XEP-0174]. In addition, XML streams can be
established end-to-end over any reliable transport, including
extensions to XMPP itself; however, such methods are out of scope
for this specification.
The following paragraphs describe the responsibilities of clients and
servers on the network.
A CLIENT is an entity that establishes an XML stream with a server by
authenticating using the credentials of a local account and that then
completes resource binding (Section 8) in order to enable delivery of
XML stanzas between the server and the client over the negotiated
stream. The client then uses XMPP to communicate with its server,
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other clients, and any other entities on the network, where the
server is responsible for delivering stanzas to local entities or
routing them to remote entities. Multiple clients can connect
simultaneously to a server on behalf of the same local account, where
each client is differentiated by the resourcepart of an XMPP address
(e.g., <localpart@domain/home> vs. <localpart@domain/work>), as
defined under Section 3 and Section 8.
A SERVER is an entity whose primary responsibilities are to:
o Manage XML streams (Section 5) with local clients and deliver XML
stanzas (Section 9) to those clients over the negotiated streams;
this includes responsibility for ensuring that a client needs to
authenticate with the server before being granted access to the
XMPP network.
o Subject to local service policies on server-to-server
communication, manage XML streams (Section 5) with remote servers
and route XML stanzas (Section 9) to those servers over the
negotiated streams.
Depending on the application, the secondary responsibilities of an
XMPP server can include:
o Storing XML data that is used by clients (e.g., contact lists for
users of XMPP-based instant messaging and presence applications as
defined in [XMPP-IM]); in this case, the relevant XML stanza is
handled directly by the server itself on behalf of the client and
is not routed to a remote server or delivered to a local entity.
o Hosting local services that also use XMPP as the basis for
communication but that provide additional functionality beyond
that defined in this document or in [XMPP-IM]; examples include
multi-user conferencing services as specified in [XEP-0045] and
publish-subscribe services as specified in [XEP-0060].
3. Addresses3.1. Overview
An ENTITY is anything that is network-addressable and that can
communicate using XMPP. For historical reasons, the native address
of an XMPP entity is called a JABBER IDENTIFIER or JID. A valid JID
contains a set of ordered elements formed of an XMPP localpart,
domainpart, and resourcepart.
The syntax for a JID is defined as follows using the Augmented
Backus-Naur Form as specified in [ABNF].
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jid = [ localpart "@" ] domain [ "/" resource ]
localpart = 1*(nodepoint)
; a "nodepoint" is a UTF-8 encoded Unicode code
; point that satisfies the Nodeprep profile of
; stringprep
domain = fqdn / address-literal
fqdn = *(ldhlabel ".") toplabel
ldhlabel = letdig [*61(ldh) letdig]
toplabel = ALPHA *61(ldh) letdig
letdig = ALPHA / DIGIT
ldh = ALPHA / DIGIT / "-"
address-literal = IPv4address / IPv6address
; the "IPv4address" and "IPv6address" rules are
; defined in RFC 3986
resource = 1*(resourcepoint)
; a "resourcepoint" is a UTF-8 encoded Unicode
; code point that satisfies the Resourceprep
; profile of stringprep
All JIDs are based on the foregoing structure. One common use of
this structure is to identify a messaging and presence account, the
server that hosts the account, and a connected resource (e.g., a
specific device) in the form of <localpart@domain/resource>.
However, localparts other than clients are possible; for example, a
specific chat room offered by a multi-user conference service (see
[XEP-0045]) could be addressed as <room@service> (where "room" is the
name of the chat room and "service" is the hostname of the multi-user
conference service) and a specific occupant of such a room could be
addressed as <room@service/nick> (where "nick" is the occupant's room
nickname). Many other JID types are possible (e.g., <domain/
resource> could be a server-side script or service).
Each allowable portion of a JID (localpart, domainpart, and
resourcepart) MUST NOT be more than 1023 bytes in length, resulting
in a maximum total size (including the '@' and '/' separators) of
3071 bytes.
Note: While the format of a JID is consistent with [URI], an
entity's address on an XMPP network MUST be represented as a JID
(without a URI scheme) and not a [URI] or [IRI] as specified in
[XMPP-URI]; the latter specification is provided only for
identification and interaction outside the context of the XMPP
wire protocol itself.
3.2. Domain Identifier
The DOMAINPART of a JID is that portion after the '@' character (if
any) and before the '/' character (if any); it is the primary
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identifier and is the only REQUIRED element of a JID (a mere
domainpart is a valid JID). Typically a domainpart identifies the
"home" server to which clients connect for XML routing and data
management functionality. However, it is not necessary for an XMPP
domainpart to identify an entity that provides core XMPP server
functionality (e.g., a domainpart can identity an entity such as a
multi-user conference service, a publish-subscribe service, or a user
directory).
Note: A single server can service multiple domainparts, i.e.,
multiple local domains; this is typically referred to as virtual
hosting.
The domainpart for every server or service that will communicate over
a network SHOULD be a fully qualified domain name (see [DNS]); while
the domainpart MAY be either an Internet Protocol (IPv4 or IPv6)
address or a text label that is resolvable on a local network
(commonly called an "unqualified hostname"), it is possible that
domainparts that are IP addresses will not be acceptable to other
services for the sake of interdomain communication. Furthermore,
domainparts that are unqualified hostnames MUST NOT be used on public
networks but MAY be used on private networks.
Note: If the domainpart includes a final character considered to
be a label separator (dot) by [IDNA] or [DNS], this character MUST
be stripped from the domainpart before the JID of which it is a
part is used for the purpose of routing an XML stanza, comparing
against another JID, or constructing an [XMPP-URI]; in particular,
the character MUST be stripped before any other canonicalization
steps are taken, such as application of the [NAMEPREP] profile of
[STRINGPREP] or completion of the ToASCII operation as described
in [IDNA].
A domainpart MUST be an "internationalized domain name" as defined in
[IDNA], that is, "a domain name in which every label is an
internationalized label". When preparing a text label (consisting of
a sequence of Unicode code points) for representation as an
internationalized label in the process of constructing an XMPP
domainpart or comparing two XMPP domainparts, an application MUST
ensure that for each text label it is possible to apply without
failing the ToASCII operation specified in [IDNA] with the
UseSTD3ASCIIRules flag set (thus forbidding ASCII code points other
than letters, digits, and hyphens). If the ToASCII operation can be
applied without failing, then the label is an internationalized
label. An internationalized domain name (and therefore an XMPP
domainpart) is constructed from its constituent internationalized
labels by following the rules specified in [IDNA].
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Note: The ToASCII operation includes application of the [NAMEPREP]
profile of [STRINGPREP] and encoding using the algorithm specified
in [PUNYCODE]; for details, see [IDNA]. Although the output of
the ToASCII operation is not used in XMPP, it MUST be possible to
apply that operation without failing.
In the terms of IDNA2008 [IDNA-DEFS], the domainpart of a JID is a
"domain name slot".
3.3. Localpart
The LOCALPART of a JID is an optional identifier placed before the
domainpart and separated from the latter by the '@' character.
Typically a localpart uniquely identifies the entity requesting and
using network access provided by a server (i.e., a local account),
although it can also represent other kinds of entities (e.g., a chat
room associated with a multi-user conference service). The entity
represented by an XMPP localpart is addressed within the context of a
specific domain.
A localpart MUST NOT be zero bytes in length and, as for all portions
of a JID, MUST NOT be more than 1023 bytes in length.
A localpart MUST be formatted such that the Nodeprep profile of
[STRINGPREP] can be applied without failing (see Appendix A). Before
comparing two localparts, an application MUST first ensure that the
Nodeprep profile has been applied to each identifier (the profile
need not be applied each time a comparison is made, as long as it has
been applied before comparison).
3.4. Resourcepart
The resourcepart of a JID is an optional identifier placed after the
domainpart and separated from the latter by the '/' character. A
resourcepart can modify either a <localpart@domain> address or a mere
<domain> address. Typically a resourcepart uniquely identifies a
specific connection (e.g., a device or location) or object (e.g., a
participant in a multi-user conference room) belonging to the entity
associated with an XMPP localpart at a local domain.
When an XMPP address does not include a resourcepart (i.e., when it
is of the form <domain> or <localpart@domain>), it is referred to as
a BARE JID. When an XMPP address includes a resourcepart (i.e., when
it is of the form <domain/resource> or <localpart@domain/resource>),
is referred to as a FULL JID.
A resourcepart MUST NOT be zero bytes in length and, as for all
portions of a JID, MUST NOT be more than 1023 bytes in length.
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A resourcepart MUST be formatted such that the Resourceprep profile
of [STRINGPREP] can be applied without failing (see Appendix B).
Before comparing two resourceparts, an application MUST first ensure
that the Resourceprep profile has been applied to each identifier
(the profile need not be applied each time a comparison is made, as
long as it has been applied before comparison).
Note: For historical reasons, the term "resource identifier" is
often used in XMPP to refer to the optional portion of an XMPP
address that follows the domainpart and the "/" separator
character; to help prevent confusion between an XMPP "resource
identifier" and the meanings of "resource" and "identifier"
provided in Section 1.1 of [URI], this specification typically
uses the term "resourcepart" instead of "resource identifier" (as
in RFC 3920).
XMPP entities SHOULD consider resourceparts to be opaque strings and
SHOULD NOT impute meaning to any given resourcepart. In particular,
the use of the '/' character as a separator between the domainpart
and the resourcepart does not imply that XMPP addresses are
hierarchical in the way that, say, HTTP addresses are hierarchical;
thus for example an XMPP address of the form
<localpart@domain/foo/bar> does not identify a resource "bar" that
exists below a resource "foo" in a hierarchy of resources associated
with the entity "localpart@domain".
3.5. Determination of Addresses
After the parties to an XML stream have completed the appropriate
aspects of stream negotiation (typically SASL negotiation (Section 7)
and, if appropriate, resource binding (Section 8)) the receiving
entity for a stream MUST determine the initiating entity's JID.
For server-to-server communication, the initiating server's JID MUST
be the authorization identity (as defined by [SASL]), either (1) as
directly communicated by the initiating server during SASL
negotiation (Section 7) or (2) as derived by the receiving server
from the authentication identity if no authorization identity was
specified during SASL negotiation (Section 7). (For information
about the determination of addresses in the absence of SASL
negotiation when the older server dialback protocol is used, see
[XEP-0220].)
For client-to-server communication, the client's bare JID
(<localpart@domain>) MUST be the authorization identity (as defined
by [SASL]), either (1) as directly communicated by the client during
SASL negotiation (Section 7) or (2) as derived by the server from the
authentication identity if no authorization identity was specified
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during SASL negotiation (Section 7). The resourcepart of the full
JID (<localpart@domain/resource>) MUST be the resource negotiated by
the client and server during resource binding (Section 8).
The receiving entity MUST ensure that the resulting JID (including
localpart, domainpart, resourcepart, and separator characters)
conforms to the rules and formats defined earlier in this section; to
meet this restriction, the receiving entity MAY replace the JID sent
by the initiating entity with the canonicalized JID as determined by
the receiving entity.
4. TCP Binding4.1. Scope
As XMPP is defined in this specification, an initiating entity
(client or server) MUST open a Transmission Control Protocol [TCP]
connection at the receiving entity (server) before it negotiates XML
streams with the receiving entity. The parties then maintain that
TCP connection for as long as the XML streams are in use. The rules
specified in the following sections apply to the TCP binding.
4.2. Hostname Resolution
Before opening the TCP connection, the initiating entity first MUST
resolve the Domain Name System (DNS) hostname associated with the
receiving entity and determine the appropriate TCP port for
communication with the receiving entity. The process is:
1. Attempt to resolve the hostname using (a) a [DNS-SRV] Service of
"xmpp-client" (for client-to-server connections) or "xmpp-server"
(for server-to-server connections) and (b) a Proto of "tcp",
resulting in resource records such as "_xmpp-
client._tcp.example.net." or "_xmpp-server._tcp.im.example.com.".
The result of the SRV lookup will be one or more combinations of
a port and hostname, which hostnames the initiating entity MUST
resolve according to returned SRV record weight (if the result of
the SRV lookup is a single RR with a Target of ".", i.e. the root
domain, the initiating entity MUST abort SRV processing but
SHOULD attempt a fallback resolution as described below). The
initiating entity uses the IP address(es) from the first
successfully resolved hostname (with the corresponding port
number returned by the SRV lookup) in order to connect to the
receiving entity. If the initiating entity fails to connect
using one of the IP addresses, the initiating entity uses the
next resolved IP address to connect. If the initiating entity
fails to connect using all resolved IP addresses, then the
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initiating entity repeats the process of resolution and
connection for the next hostname returned by the SRV lookup.
2. If the SRV lookup aborts or fails, the fallback SHOULD be a
normal IPv4 or IPv6 address record resolution to determine the IP
address, where the port used is the "xmpp-client" port of 5222
for client-to-server connections or the "xmpp-server" port 5269
for server-to-server connections.
3. For client-to-server connections, the fallback MAY be a [DNS-TXT]
lookup for alternative connection methods, for example as
described in [XEP-0156].
Note: If the initiating entity has been explicitly configured to
associate a particular hostname (and potentially port) with the
original hostname of the receiving entity (say, to "hardcode" an
association between an original hostname of example.net and a
configured hostname and of webcm.example.com:80), the initiating
entity SHALL use the configured name instead of performing the
foregoing resolution process on the original name.
Note: Many XMPP servers are implemented in such a way that they
can host additional services (beyond those defined in this
specification and [XMPP-IM]) at hostnames that are subdomains of
the hostname of the main XMPP service (e.g.,
conference.example.net for a [XEP-0045] service associated with
the example.net XMPP service) or subdomains of the first-level
domain of the underlying host (e.g., muc.example.com for a
[XEP-0045] service associated with the im.example.com XMPP
service). If an entity from a remote domain wishes to use such
additional services, it would generate an appropriate XML stanza
and the remote domain itself would attempt to resolve the
service's hostname via an SRV lookup on resource records such as
"_xmpp-server._tcp.conference.example.net." or "_xmpp-
server._tcp.muc.example.com.". Therefore if a service wishes to
enable entities from remote domains to access these additional
services, it needs to advertise the appropriate "_xmpp-server" SRV
records in addition to the "_xmpp-server" record for its main XMPP
service.
4.3. Client-to-Server Communication
Because a client is subordinate to a server and therefore a client
authenticates to the server but the server does not necessarily
authenticate to the client, it is necessary to have only one TCP
connection between client and server. Thus the server MUST allow the
client to share a single TCP connection for XML stanzas sent from
client to server and from server to client (i.e., the inital stream
and response stream as specified under Section 5).
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Because two servers are peers and therefore each peer MUST
authenticate with the other, the servers MUST use two TCP
connections: one for XML stanzas sent from the first server to the
second server and another (initiated by the second server) for XML
stanzas from the second server to the first server.
This rule applies only to XML stanzas (Section 9). Therefore during
STARTTLS negotiation (Section 6) and SASL negotiation (Section 7) the
servers would use one TCP connection, but after stream setup that TCP
connection would be used only for the initiating server to send XML
stanzas to the receiving server. In order for the receiving server
to send XML stanzas to the initiating server, the receiving server
would need to reverse the roles and negotiate an XML stream from the
receiving server to the initiating server.
4.5. Reconnection
It can happen that an XMPP server goes offline while servicing TCP
connections from local clients and from other servers. Because the
number of such connections can be quite large, the reconnection
algorithm employed by entities that seek to reconnect can have a
significant impact on software and network performance. The
following guidelines are RECOMMENDED:
o The time to live (TTL) specified in Domain Name System records
MUST be honored, even if DNS results are cached; if the TTL has
not expired, an entity that seeks to reconnect MUST NOT re-resolve
the server hostname before reconnecting.
o The time that expires before an entity first seeks to reconnect
MUST be randomized (e.g., so that all clients do not attempt to
reconnect exactly 30 seconds after being disconnected).
o If the first reconnection attempt does not succeed, an entity MUST
back off increasingly on the time between subsequent reconnection
attempts, e.g. in accordance with the exponential backoff
principle.
Note: Because it is possible that a disconnected entity cannot
determine the cause of disconnection (e.g., because there was no
explicit stream error) or does not need a new stream for immediate
communication (e.g., because the stream was idle and therefore
timed out), it SHOULD NOT assume that is needs to reconnect
immediately.
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The use of long-lived TCP connections in XMPP implies that the
sending of XML stanzas over XML streams can be unreliable, since the
parties to a long-lived TCP connection might not discover a
connectivity disruption in a timely manner. At the XMPP application
layer, long connectivity disruptions can result in undelivered
stanzas. Although the core XMPP technology defined in this
specification does not contain features to overcome this lack of
reliability, there exist XMPP extensions for doing so (e.g.,
[XEP-0198]).
4.7. Other Bindings
There is no necessary coupling of an XML stream to a TCP connection.
For example, two entities could connect to each other via another
transport, such as [HTTP] as specified in [XEP-0124] and [XEP-0206].
Although this specification neither encourages nor discourages other
bindings, it defines only a binding of XMPP to TCP.
5. XML Streams5.1. Overview
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
between presence-aware entities: XML streams and XML stanzas. These
terms are defined as follows.
Definition of XML Stream: An XML STREAM is a container for the
exchange of XML elements between any two entities over a network.
The start of an XML stream is denoted unambiguously by an opening
STREAM HEADER (i.e., an XML <stream> tag with appropriate
attributes and namespace declarations), while the end of the XML
stream is denoted unambiguously by a closing XML </stream> tag.
During the life of the stream, the entity that initiated it can
send an unbounded number of XML elements over the stream, either
elements used to negotiate the stream (e.g., to complete TLS
negotiation (Section 6) or SASL negotiation (Section 7)) or XML
stanzas. The INITIAL STREAM is negotiated from the initiating
entity (typically a client or server) to the receiving entity
(typically a server), and can be seen as corresponding to the
initiating entity's "connection" or "session" with the receiving
entity. The initial stream enables unidirectional communication
from the initiating entity to the receiving entity; in order to
enable information exchange from the receiving entity to the
initiating entity, the receiving entity MUST negotiate a stream in
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the opposite direction (the RESPONSE STREAM).
Definition of XML Stanza: An XML STANZA is a discrete semantic unit
of structured information that is sent from one entity to another
over an XML stream, and is the basic unit of meaning in XMPP. An
XML stanza exists at the direct child level of the root <stream/>
element; the start of any XML stanza is denoted unambiguously by
the element start tag at depth=1 of the XML stream (e.g.,
<presence>), and the end of any XML stanza is denoted
unambiguously by the corresponding close tag at depth=1 (e.g.,
</presence>). The only XML stanzas defined herein are the
<message/>, <presence/>, and <iq/> elements qualified by the
default namespace for the stream, as described under Section 9;
for example, an XML element sent for the purpose of TLS
negotiation (Section 6) or SASL negotiation (Section 7) is not
considered to be an XML stanza, nor is a stream error or a stream
feature. An XML stanza MAY contain child elements (with
accompanying attributes, elements, and XML character data) as
necessary in order to convey the desired information, which MAY be
qualified by any XML namespace (see [XML-NAMES] as well as
Section 9.4 herein).
Consider the example of a client's connection to a server. In order
to connect to a server, a client initiates an XML stream by sending a
stream header to the server, optionally preceded by a text
declaration specifying the XML version and the character encoding
supported (see Section 12.5 and Section 12.6). Subject to local
policies and service provisioning, the server then replies with a
second XML stream back to the client, again optionally preceded by a
text declaration. Once the client has completed SASL negotiation
(Section 7) and resource binding (Section 8), the client can send an
unbounded number of XML stanzas over the stream. When the client
desires to close the stream, it simply sends a closing </stream> tag
to the server as further described under Section 5.3.
In essence, then, an XML stream acts as an envelope for all the XML
stanzas sent during a connection. We can represent this in a
simplistic fashion as follows.
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+--------------------+
| <stream> |
|--------------------|
| <presence> |
| <show/> |
| </presence> |
|--------------------|
| <message to='foo'> |
| <body/> |
| </message> |
|--------------------|
| <iq to='bar'> |
| <query/> |
| </iq> |
|--------------------|
| <iq from='bar'> |
| <query/> |
| </iq> |
|--------------------|
| [ ... ] |
|--------------------|
| </stream> |
+--------------------+
Note: Those who are accustomed to thinking of XML in a document-
centric manner might view a client's connection to a server as
consisting of two open-ended XML documents: one from the client to
the server and one from the server to the client. On this
analogy, the two XML streams can be considered equivalent to two
"documents" (matching production [1] content of [XML]) that are
built up through the accumulation of XML stanzas, the root
<stream/> element can be considered equivalent to the "document
entity" for each "document" (as described in Section 4.8 of
[XML]), and the XML stanzas sent over the streams can be
considered equivalent to "fragments" of the "documents" as
described in [XML-FRAG]. However, this perspective is merely an
analogy; XMPP does not deal in documents and fragments but in
streams and stanzas.
The remainder of this section defines the following aspects of XML
streams:
o The stream negotation process
o How to close a stream
o The XML attributes of a stream
o The XML namespaces of a stream
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Internet-Draft XMPP Core March 20105.2. Stream Negotiation5.2.1. Overview
Because the receiving entity for a stream acts as a gatekeeper to the
domains it services, it imposes certain conditions for connecting as
a client or as a peer server. At a minimum, the initiating entity
needs to authenticate with the receiving entity before it is allowed
to send stanzas to the receiving entity, typically using SASL as
described under Section 7. However, the receiving entity can
consider conditions other than authentication to be mandatory, such
as encryption using TLS as described under Section 6. The receiving
entity informs the initiating entity about such conditions by
communicating STREAM FEATURES: the set of particular protocol
interactions that are mandatory for the initiating entity to complete
before the receiving entity will accept XML stanzas from the
initiating entity (e.g., authentication), as well as any protocol
interactions that are voluntary but that might improve the handling
of an XML stream (e.g., establishment of application-layer
compression).
The existence of conditions for connecting implies that streams need
to be negotiated. The order of layers (TCP, then TLS, then SASL,
then XMPP; see Section 14.5) implies that stream negotiation is a
multi-stage process. Further structure is imposed by two factors:
(1) a given stream feature might be made available only to certain
entities or only after certain other features have been negotiated
(e.g., resource binding is made available only after SASL
authentication), and (2) stream features can be either mandatory-to-
negotiate or voluntary-to-negotiate. Finally, for security reasons
the parties to a stream need to discard knowledge that they gained
during the negotiation process after successfully completing the
protocol interactions defined for certain features (e.g., TLS in all
cases and SASL in the case when a security layer might be
established); this is done by flushing the old stream context and
exchanging new stream headers over the existing TCP connection.
5.2.2. Stream Features Format
If the initiating entity includes the 'version' attribute set to a
value of at least "1.0" in the initial stream header, after sending
the response stream header the receiving entity MUST send a
<features/> child element (prefixed by the streams namespace prefix)
to the initiating entity in order to announce any conditions for
continuation of the stream negotiation process. Each condition takes
the form of a child element of the <features/> element, qualified by
a namespace that is different from the streams namespace and the
default namespace.
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If a particular stream feature is or can be mandatory-to-negotiate,
the definition of that feature needs to either declare that the
feature is always mandatory-to-negotiate (e.g., this is true of
resource binding for XMPP clients) or specify a way for the receiving
entity to flag the feature as mandatory-to-negotiate for this
interaction (e.g., this is done for TLS by including an empty
<required/> element in the advertisement for that stream feature).
For security reasons, certain stream features necessitate the
initiating entity to send a new initial stream header upon successful
negotiation of the feature (e.g., TLS in all cases and SASL in the
case when a security layer might be established). If this is true of
a given stream feature, the definition of that feature needs to
declare that a stream restart is expected after negotiation of the
feature.
A <features/> element that contains at least one mandatory feature
indicates that the stream negotiation is not complete and that the
initiating entity MUST negotiate further features.
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
A <features/> element MAY contain more than one mandatory feature.
This means that the initiating entity can choose among the mandatory
features. For example, perhaps a future technology will perform
roughly the same function as TLS, so the receiving entity might
advertise support for both TLS and the future technology.
A <features/> element that contains only voluntary features indicates
that the stream negotiation is complete and that the initiating
entity is cleared to send XML stanzas, but that the initiating entity
MAY negotiate further features if desired.
R: <stream:features>
<session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
<compression xmlns='http://jabber.org/features/compress'>
<method>zlib</method>
<method>lzw</method>
</compression>
</stream:features>
A <features/> element that contains both mandatory and voluntary
features indicates that the negotiation is not complete but that the
initiating entity MAY complete the voluntary feature(s) before it
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attempts to negotiate the mandatory feature(s).
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<compression xmlns='http://jabber.org/features/compress'>
<method>zlib</method>
<method>lzw</method>
</compression>
</stream:features>
An empty <features/> element indicates that the stream negotiation is
complete and that the initiating entity is cleared to send XML
stanzas.
R: <stream:features/>
Note: The order of child elements contained in any given
<features/> element is not significant.
5.2.3. Restarts
On successful negotiation of a feature that necessitates a stream
restart, both parties MUST consider the previous stream to be
replaced but MUST NOT terminate the underlying TCP connection;
instead, the parties MUST reuse the existing connection, which might
be in a new state (e.g., encrypted as a result of TLS negotiation).
The initiating entity then MUST send a new initial stream header,
which SHOULD be preceded by an XML declaration as described under
Section 12.5. When the receiving entity receives the new initial
stream header, it MUST generate a new stream ID (instead of re-using
the old stream ID) before sending a new response stream header (which
SHOULD be preceded by an XML declaration as described under
Section 12.5).
For the sake of backward compatibility, the receiving entity MUST
accept stream restarts at any stage in the stream negotiation process
even if the receiving entity has not indicated that a stream restart
is mandatory at that stage.
5.2.4. Resending Features
After completing negotiation of any stream feature (even stream
features that do not necessitate a stream restart), the receiving
entity MUST send an updated list of stream features to the initiating
entity, where the list MAY be empty if there are no further features
to be advertised.
At any time after stream establishment and before closing of the
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stream, the receiving entity MAY send additional or modified stream
feature advertisements to the initiating entity (e.g., because a new
feature has been enabled).
5.2.5. Completion of Stream Negotiation
The receiving entity indicates completion of the stream negotiation
process by sending to the initiating entity either an empty
<features/> element or a <features/> element that contains only
voluntary features. After doing so, the receiving entity MUST NOT
send additional stream features to the initiating entity (if the
receiving entity has new features to offer, it can simply close the
stream using a <reset/> stream error and then advertise the new
features when the initiating entity reconnects, preferably closing
existing streams in a staggered way so that not all of the initiating
entities reconnect at once). Once stream negotiation is complete,
the initiating entity is cleared to send XML stanzas over the stream
for as long as the stream is maintained by both parties.
The initiating entity MUST NOT attempt to send XML stanzas
(Section 9) to entities other than itself (i.e., the client's
connected resource or any other authenticated resource of the
client's account) or the server until stream negotiation has been
completed. However, if it does attempt to do so, the receiving
entity MUST NOT accept such stanzas and MUST return a <not-
authorized/> stream error. This rule applies to XML stanzas only
(i.e., <message/>, <presence/>, and <iq/> elements qualified by the
default namespace) and not to XML elements used for stream
negotiation (e.g., elements used to complete TLS negotiation
(Section 6) or SASL negotiation (Section 7)).
5.2.6. State Chart
We summarize the foregoing rules in the following non-normative state
chart for the stream negotiation process, presented from the
perspective of the initiating entity.
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Internet-Draft XMPP Core March 20105.3. Closing a Stream
An XML stream between two entities can be closed because a stream
error has occurred or in some cases in the absence of an error.
Where feasible, it is preferable to close a stream only if a stream
error has occurred.
A stream is closed by sending a closing </stream> tag over the TCP
connection.
S: </stream:stream>
After an entity sends a closing stream tag, it MUST NOT send further
data over that stream.
5.3.1. With Stream Error
If a stream error has occurred, the entity that detects the error
MUST close the stream as described under Section 5.6.1.
5.3.2. Without Stream Error
At any time after XML streams have been negotiated between two
entities, either entity MAY close its stream to the other party in
the absence of a stream error by sending a closing stream tag.
P: </stream:stream>
The entity that sends the closing stream tag SHOULD wait for the
other party to also close its stream.
S: </stream:stream>
However, the entity that sends the first closing stream tag MAY
consider both streams to be void if the other party does not send its
closing stream tag within a reasonable amount of time (where the
definition of "reasonable" is a matter of implementation or
deployment).
After the entity that sent the first closing stream tag receives a
reciprocal closing stream tag from the other party (or if it
considers the stream to be void after a reasonable amount of time),
it MUST terminate the underlying TCP connection or connections.
5.3.3. Handling of Idle Streams
An XML stream can become idle, i.e., neither entity has sent XMPP
traffic over the stream for some period of time. The idle timeout
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period is a matter of implementation and local service policy;
however, it is RECOMMENDED to be liberal in accepting idle streams,
since experience has shown that doing so improves the reliability of
communications over XMPP networks. In particular, it is typically
more efficient to maintain a stream between two servers than it is to
aggressively timeout such a stream, especially with regard to
synchronization of presence information as described in [XMPP-IM];
therefore it is RECOMMENDED to maintain such a stream since
experience has shown that server-to-server streams are cyclical and
typically need to be re-established every 24 to 48 hours if they are
timed out.
An XML stream can appear idle at the XMPP level because the
underlying TCP connection has become idle (e.g., a client's network
connection has been lost). One common method for preventing a TCP
connection from going idle or for detecting an idle TCP connection is
to send a space character (U+0020) over the TCP connection between
XML stanzas, which is allowed for XML streams as described under
Section 12.7; the sending of such a space character is properly
called a WHITESPACE KEEPALIVE (although the term "whitespace ping" is
often used, in fact it is not a ping since no "pong" is possible).
Other connection-testing methods include the application-level pings
described in [XEP-0199] and the more comprehensive stream management
protocol described in [XEP-0198]. Implementers are advised to
support whichever connection-testing methods they deem appropriate,
but to carefully weigh the network impact of such methods against the
benefits of discovering idle streams in a timely manner. The length
of time between the use of any particular connection test is a matter
of implementation and local service policy; however, it is
RECOMMENDED that any such test be performed not more than once every
60 seconds.
To close an idle stream with a local client or remote server, a
server MUST close the stream without error as explained under
Section 5.3.2.
5.4. Stream Attributes
The attributes of the root <stream/> element are defined in the
following sections.
Note: The attributes of the root <stream/> element are not
prepended by a namespace prefix because, as explained in
[XML-NAMES], "[d]efault namespace declarations do not apply
directly to attribute names; the interpretation of unprefixed
attributes is determined by the element on which they appear."
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The 'from' attribute communicates an XMPP identity of the entity
sending the stream element.
Note: It is possible for an entity to have more than one XMPP
identity (e.g., in the case of a server that provides virtual
hosting). It is also possible that an entity does not know the
XMPP identity of the principal controlling the entity (e.g.,
because the XMPP identity is assigned at a level other than the
XMPP application layer, as in the General Security Service
Application Program Interface [GSS-API]).
For initial stream headers in client-to-server communication, if the
client knows the XMPP identity of the principal controlling the
client (typically an account name of the form <localpart@domain>),
then it MAY include the 'from' attribute and set its value to that
identity; if not, then it MUST NOT include the 'from' attribute.
Note: Including the XMPP identity before the stream is protected via
TLS can expose that identity to eavesdroppers.
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For initial stream headers in server-to-server communication, a
server MUST include the 'from' attribute and MUST set its value to a
hostname serviced by the initiating entity.
I: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in both client-to-server and server-to-
server communication, the receiving entity MUST include the 'from'
attribute and MUST set its value to a hostname serviced by the
receiving entity (which MAY be a hostname other than that specified
in the 'to' attribute of the initial stream header).
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R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Whether or not the 'from' attribute is included, each entity MUST
verify the identity of the other entity before exchanging XML stanzas
with it (see Section 14.3 and Section 14.4).
Note: It is possible that implementations based on the predecessor
to this specification will not include the 'from' address on
stream headers; an entity SHOULD be liberal in accepting such
stream headers.
5.4.2. to
For initial stream headers in both client-to-server and server-to-
server communication, the initiating entity MUST include the 'to'
attribute and MUST set its value to a hostname that the initiating
entity knows or expects the receiving entity to service.
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in client-to-server communication, if the
client included a 'from' attribute in the initial stream header then
the server MUST include a 'to' attribute in the response stream
header and MUST set its value to the bare JID specified in the 'from'
attribute of the initial stream header. If the client did not
include a 'from' attribute in the initial stream header then the
server MUST NOT include a 'to' attribute in the response stream
header.
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R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers in server-to-server communication, the
receiving entity MUST include a 'to' attribute in the response stream
header and MUST set its value to the hostname specified in the 'from'
attribute of the initial stream header.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
Whether or not the 'to' attribute is included, each entity MUST
verify the identity of the other entity before exchanging XML stanzas
with it (see Section 14.3 and Section 14.4).
Note: It is possible that implementations based on the predecessor
to this specification will not include the 'to' address on stream
headers; an entity SHOULD be liberal in accepting such stream
headers.
5.4.3. id
The 'id' attribute communicates a unique identifier for the stream.
This identifier is called a STREAM ID. The stream ID MUST be
generated by the receiving entity when it sends a response stream
header, MUST BE unique within the receiving application (normally a
server), and MUST be both unpredictable and nonrepeating because it
can be security-critical (see [RANDOM] for recommendations regarding
randomness for security purposes).
For initial stream headers, the initiating entity MUST NOT include
the 'id' attribute; however, if the 'id' attribute is included, the
receiving entity MUST silently ignore it.
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For response stream headers, the receiving entity MUST include the
'id' attribute.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
5.4.4. xml:lang
The 'xml:lang' attribute communicates an entity's preferred or
default language for any human-readable XML character data to be sent
over the stream. The syntax of this attribute is defined in Section2.12 of [XML]; in particular, the value of the 'xml:lang' attribute
MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of
[XML]) and MUST conform to the language identifier format defined in
[LANGTAGS].
For initial stream headers, the initiating entity SHOULD include the
'xml:lang' attribute.
I: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
For response stream headers, the receiving entity MUST include the
'xml:lang' attribute. If the initiating entity included an 'xml:
lang' attribute in its initial stream header and the receiving entity
supports that language in the human-readable XML character data that
it generates and sends to the initiating entity (e.g., in the <text/>
element for stream and stanza errors), the value of the 'xml:lang'
attribute MUST be an identifier for the initiating entity's preferred
language; if the receiving entity supports a language that closely
matches the initiating entity's preferred language (e.g., "de"
instead of "de-CH"), then the value of the 'xml:lang' attribute
SHOULD be the identifier for the matching language but MAY be the
identifier for the default language of the receiving entity; if the
receiving entity does not support the initiating entity's preferred
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language or a closely matching language (or the initiating entity did
not include the 'xml:lang' attribute in its initial stream header),
then the value of the 'xml:lang' attribute MUST be the identifier for
the default language of the receiving entity.
R: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
If the initiating entity included the 'xml:lang' attribute in its
initial stream header, the receiving entity SHOULD remember that
value as the default xml:lang for all stanzas sent by the initiating
entity. As described under Section 9.1.5, the initiating entity MAY
include the 'xml:lang' attribute in any XML stanzas it sends over the
stream. If the initiating entity does not include the 'xml:lang'
attribute in any such stanza, the receiving entity SHOULD add the
'xml:lang' attribute to the stanza, whose value MUST be the
identifier for the language preferred by the initiating entity (even
if the receiving entity does not support that language for human-
readable XML character data it generates and sends to the initiating
entity, such as in stream or stanza errors). If the initiating
entity includes the 'xml:lang' attribute in any such stanza, the
receiving entity MUST NOT modify or delete it.
5.4.5. version
The inclusion of the version attribute set to a value of at least
"1.0" signals support for the stream-related protocols defined in
this specification, including (TLS negotiation (Section 6), SASL
negotiation (Section 7), Section 5.2.2, and stream errors
(Section 5.6).
The version of XMPP specified herein is "1.0"; in particular, XMPP
1.0 encapsulates the stream-related protocols as well as the basic
semantics of the three defined XML stanza types (<message/>,
<presence/>, and <iq/>).
The numbering scheme for XMPP versions is "<major>.<minor>". The
major and minor numbers MUST be treated as separate integers and each
number MAY be incremented higher than a single digit. Thus, "XMPP
2.4" would be a lower version than "XMPP 2.13", which in turn would
be lower than "XMPP 12.3". Leading zeros (e.g., "XMPP 6.01") MUST be
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ignored by recipients and MUST NOT be sent.
The major version number will be incremented only if the stream and
stanza formats or obligatory actions have changed so dramatically
that an older version entity would not be able to interoperate with a
newer version entity if it simply ignored the elements and attributes
it did not understand and took the actions specified in the older
specification.
The minor version number will be incremented only if significant new
capabilities have been added to the core protocol (e.g., a newly
defined value of the 'type' attribute for message, presence, or IQ
stanzas). The minor version number MUST be ignored by an entity with
a smaller minor version number, but MAY be used for informational
purposes by the entity with the larger minor version number (e.g.,
the entity with the larger minor version number would simply note
that its correspondent would not be able to understand that value of
the 'type' attribute and therefore would not send it).
The following rules apply to the generation and handling of the
'version' attribute within stream headers:
1. The initiating entity MUST set the value of the 'version'
attribute in the initial stream header to the highest version
number it supports (e.g., if the highest version number it
supports is that defined in this specification, it MUST set the
value to "1.0").
2. The receiving entity MUST set the value of the 'version'
attribute in the response stream header to either the value
supplied by the initiating entity or the highest version number
supported by the receiving entity, whichever is lower. The
receiving entity MUST perform a numeric comparison on the major
and minor version numbers, not a string match on
"<major>.<minor>".
3. If the version number included in the response stream header is
at least one major version lower than the version number included
in the initial stream header and newer version entities cannot
interoperate with older version entities as described, the
initiating entity SHOULD generate an <unsupported-version/>
stream error.
4. If either entity receives a stream header with no 'version'
attribute, the entity MUST consider the version supported by the
other entity to be "0.9" and SHOULD NOT include a 'version'
attribute in the response stream header.
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Internet-Draft XMPP Core March 20105.4.6. Summary of Stream Attributes
The following table summarizes the attributes of the root <stream/>
element.
+----------+--------------------------+-------------------------+
| | initiating to receiving | receiving to initiating |
+----------+--------------------------+-------------------------+
| to | JID of receiver | JID of initiator |
| from | JID of initiator | JID of receiver |
| id | silently ignored | stream identifier |
| xml:lang | default language | default language |
| version | XMPP 1.0+ supported | XMPP 1.0+ supported |
+----------+--------------------------+-------------------------+
5.5. Namespace Declarations5.5.1. Declaration of Streams Namespace
A streams namespace declaration is REQUIRED in all XML stream headers
and the name of the streams namespace MUST be
'http://etherx.jabber.org/streams'. If this rule is violated, the
entity that receives the offending stream header MUST return a stream
error to the sending entity, which SHOULD be <invalid-namespace/> but
MAY be <bad-format/>.
The element names of the <stream/> element and its <features/> and
<error/> children MUST be qualified by the streams namespace prefix
in all instances. If this rule is violated, the entity that receives
the offending element MUST return a stream error to the sending
entity, which SHOULD be <bad-format/>.
For historical reasons, an implementation MAY accept only the
'stream:' prefix for these elements. If an entity receives a stream
header with a streams namespace prefix it does not accept, it MUST
return a stream error to the sending entity, which SHOULD be <bad-
namespace-prefix/> but MAY be <bad-format/>.
5.5.2. Declaration of Default Namespace
A default namespace declaration is REQUIRED and defines the allowable
first-level children of the root stream element. This namespace
declaration MUST be the same for the initial stream and the response
stream so that both streams are qualified consistently. The default
namespace declaration applies to the stream and all first-level child
element sent within a stream unless explicitly qualified by the
streams namespace or another namespace.
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A server implementation MUST support the following two default
namespaces:
o jabber:client -- this default namespace is declared when the
stream is used for communication between a client and a server
o jabber:server -- this default namespace is declared when the
stream is used for communication between two servers
A client implementation MUST support the 'jabber:client' default
namespace.
If an implementation accepts a stream that is qualified by the
'jabber:client' or 'jabber:server' namespace, it MUST support the
common attributes (Section 9.1) and basic semantics (Section 9.2) of
all three core stanza types (message, presence, and IQ).
For historical reasons, an implementation MAY refuse to support any
other default namespaces. If an entity receives a stream header with
a default namespace it does not support, it MUST return an <invalid-
namespace/> stream error.
An implementation MUST NOT generate namespace prefixes for elements
qualified by the default namespace if the default namespace is
'jabber:client' or 'jabber:server'.
Note: The 'jabber:client' and 'jabber:server' namespaces are
nearly identical but are used in different contexts (client-to-
server communication for 'jabber:client' and server-to-server
communication for 'jabber:server'). The only difference between
the two is that the 'to' and 'from' attributes are OPTIONAL on
stanzas sent over XML streams qualified by the 'jabber:client'
namespace, whereas they are REQUIRED on stanzas sent over XML
streams qualified by the 'jabber:server' namespace.
An implementation MAY support a default namespace other than "jabber:
client" or "jabber:server". However, because such namespaces would
define applications other than XMPP, they are to be defined in
separate specifications.
5.5.3. Declaration of Other Namespaces
Because an XML stanza is the primary unit of meaning in XMPP and
because an XML stanza can be routed outside the context of the stream
in which it originated, a stream header MUST NOT include namespace
declarations for namespaces that are different from the streams
namespace and the default namespace.
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Internet-Draft XMPP Core March 20105.6. Stream Errors
The root stream element MAY contain an <error/> child element that is
prefixed by the streams namespace prefix. The error child SHALL be
sent by a compliant entity if it perceives that a stream-level error
has occurred.
5.6.1. Rules
The following rules apply to stream-level errors.
5.6.1.1. Stream Errors Are Unrecoverable
Stream-level errors are unrecoverable. Therefore, if an error occurs
at the level of the stream, the entity that detects the error MUST
send a <error/> element with an appropriate child element that
specifies the error condition and at the same time send a closing
</stream> tag.
C: <message><body></message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
The entity that generates the stream error then SHOULD immediately
terminate the underlying TCP connection, although it MAY wait until
after it receives a closing </stream> tag from the entity to which it
sent the stream error.
C: </stream:stream>
5.6.1.2. Stream Errors Can Occur During Setup
If the error is triggered by the initial stream header, the receiving
entity MUST still send the opening <stream> tag, include the <error/>
element as a child of the stream element, and send the closing
</stream> tag (preferably all at the same time).
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.1.3. Stream Errors When the Host is Unspecified or Unknown
If the initiating entity provides no 'to' attribute or provides an
unknown host in the 'to' attribute and the error occurs during stream
setup, the receiving entity SHOULD provide an authoritative hostname
in the 'from' attribute of the stream header sent before termination,
but absent such an authoritative hostname MAY instead simply populate
the response stream's 'from' attribute with the value of the initial
stream header's 'to' attribute (where the value of the 'from'
attribute MAY be empty if the initiating entity provided no 'to'
attribute).
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C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='unknown.host.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.1.4. Where Stream Errors Are Sent
When two XML streams are used between the initiating entity and the
receiving entity (one in each direction) rather than using a single
bidirectional stream, stanza errors triggered by stanzas sent over
the outbound stream are returned on the inbound stream (since they
are inbound stanzas from the perspective of the entity that sent the
triggering stanza), whereas stream errors related to the outbound
stream are returned on the outbound stream (since they are not
inbound stanzas from the perspective of the entity that sent the
triggering stanza but strictly related to the outbound stream
itself); the same is true, naturally, of any stream errors that are
related to the outbound stream but not triggered by an outbound
stanza.
5.6.2. Syntax
The syntax for stream errors is as follows, where "defined-condition"
is a placeholder for one of the conditions defined under
Section 5.6.3.
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<stream:error>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'
xml:lang='langcode'>
[ ... descriptive text ... ]
</text>]
[application-specific condition element]
</stream:error>
The <error/> element:
o MUST contain a child element corresponding to one of the defined
stream error conditions (Section 5.6.3); this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace.
o MAY contain a <text/> child element containing XML character data
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
defined namespace, and its structure is defined by that namespace
(see Section 5.6.4).
The <text/> element is OPTIONAL. If included, it MUST be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
MUST NOT be interpreted programmatically by an application. It MUST
NOT be used as the error message presented to a human user, but MAY
be shown in addition to the error message associated with the defined
condition element (and, optionally, the application-specific
condition element).
5.6.3. Defined Stream Error Conditions
The following stream-level error conditions are defined.
5.6.3.1. bad-format
The entity has sent XML that cannot be processed.
(In the following example, the client sends an XMPP message that is
not well-formed XML.)
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C: <message>
<body>No closing body tag!
</message>
S: <stream:error>
<bad-format
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
This error MAY be used instead of the more specific XML-related
errors, such as <bad-namespace-prefix/>, <invalid-xml/>, <restricted-
xml/>, <unsupported-encoding/>, and <xml-not-well-formed/>. However,
the more specific errors are RECOMMENDED.
5.6.3.2. bad-namespace-prefix
The entity has sent a namespace prefix that is unsupported, or has
sent no namespace prefix on an element that needs such a prefix (see
Section 12.2).
(In the following example, the client specifies a namespace prefix of
"foobar" for the XML streams namespace.)
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:foobar='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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Internet-Draft XMPP Core March 20105.6.3.3. conflict
The server is either (1) closing the existing stream for this entity
because a new stream has been initiated that conflicts with the
existing stream, or (2) is refusing a new stream for this entity
because allowing the new stream would conflict with an existing
stream (e.g., because the server allows only a certain number of
connections from the same IP address).
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<conflict
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.4. connection-timeout
The entity has not generated any traffic over the stream for some
period of time (configurable according to a local service policy) and
therefore the connection is being dropped.
P: <stream:error>
<connection-timeout
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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Internet-Draft XMPP Core March 20105.6.3.5. host-gone
The value of the 'to' attribute provided in the initial stream header
corresponds to a hostname that is no longer serviced by the receiving
entity.
(In the following example, the peer specifies a 'to' address of
"foo.im.example.com" when connecting to the "im.example.com" server,
but the server no longer hosts a service at that address.)
P: <?xml version='1.0'?>
<stream:stream
from='example.net'
to='foo.im.example.com'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
to='example.net'
version='1.0'
xml:lang='en'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<host-gone
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.6. host-unknown
The value of the 'to' attribute provided in the initial stream header
does not correspond to a hostname that is serviced by the receiving
entity.
(In the following example, the peer specifies a 'to' address of
"example.org" when connecting to the "im.example.com" server, but the
server knows nothing of that address.)
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S: <stream:error>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.9. invalid-from
The JID or hostname provided in a 'from' address is not a valid JID
or does not match an authorized JID or validated domain as negotiated
between servers via SASL or server dialback, or as negotiated between
a client and a server via authentication and resource binding.
(In the following example, a peer that has authenticated only as
"example.net" attempts to send a stanza from an address at
"example.org".)
P: <message from='romeo@example.org' to='juliet@im.example.com'>
<body>Neither, fair saint, if either thee dislike.</body>
</message>
S: <stream:error>
<invalid-from
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.10. invalid-id
The stream ID or server dialback ID is invalid or does not match an
ID previously provided.
(In the following example, the server dialback ID is invalid; see
[XEP-0220].)
P: <db:verify
from='example.net'
to='im.example.com'
id='unknown-id'
type='invalid'/>
S: <stream:error>
<invalid-id
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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Internet-Draft XMPP Core March 20105.6.3.11. invalid-namespace
The streams namespace name is something other than
"http://etherx.jabber.org/streams" (see Section 12.2) or the default
namespace is not supported (e.g., something other than "jabber:
client" or "jabber:server").
(In the following example, the client specifies a namespace of
'http://wrong.namespace.example.org/' for the stream.)
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://wrong.namespace.example.org/'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.12. invalid-xml
The entity has sent invalid XML over the stream to a server that
performs validation (see Section 12.4).
(In the following example, the peer attempts to send an IQ stanza of
type "subscribe" but the XML schema defines no such value for the
'type' attribute.)
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P: <iq from='example.net'
id='some-id'
to='im.example.com'
type='subscribe'>
<ping xmlns='urn:xmpp:ping'/>
</iq>
S: <stream:error>
<invalid-xml
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.13. not-authorized
The entity has attempted to send XML stanzas before the stream has
been authenticated, or otherwise is not authorized to perform an
action related to stream negotiation; the receiving entity MUST NOT
process the offending stanza before sending the stream error.
(In the following example, the client attempts to send XML stanzas
before authenticating with the server.)
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Internet-Draft XMPP Core March 20105.6.3.15. remote-connection-failed
The server is unable to properly connect to a remote entity that is
needed for authentication or authorization, such as a remote
authentication database or (in server dialback) the authoritative
server.
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<remote-connection-failed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.16. reset
The server is closing the stream because it has new (typically
security-critical) features to offer or needs to reset the stream for
some other reason (e.g., because the certificates used to establish a
secure context for the stream have expired or have been revoked
during the life of the stream).
S: <stream:error>
<reset
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
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Internet-Draft XMPP Core March 20105.6.3.19. see-other-host
The server will not provide service to the initiating entity but is
redirecting traffic to another host; the XML character data of the
<see-other-host/> element returned by the server SHOULD specify the
alternate hostname or IP address at which to connect, which SHOULD be
a valid domainpart but MAY also include a port number. When it
receives a see-other-host stream error, the initiating entity SHOULD
cleanly handle the disconnection and then reconnect to the host
specified in the <see-other-host/> element; if no port is specified,
the initiating entity SHOULD perform a [DNS-SRV] lookup on the
provided domainpart but MAY assume that it can connect to that
domainpart at the standard XMPP ports (i.e., 5222 for client-to-
server connections and 5269 for server-to-server connections).
C: <?xml version='1.0'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<stream:error>
<see-other-host
xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
[2001:41D0:1:A49b::1]:9222
</see-other-host>
</stream:error>
</stream:stream>
5.6.3.20. system-shutdown
The server is being shut down and all active streams are being
closed.
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S: <stream:error>
<system-shutdown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.21. undefined-condition
The error condition is not one of those defined by the other
conditions in this list; this error condition SHOULD be used only in
conjunction with an application-specific condition.
S: <stream:error>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
<app-error xmlns='http://example.com/ns'/>
</stream:error>
</stream:stream>
5.6.3.22. unsupported-encoding
The initiating entity has encoded the stream in an encoding that is
not supported by the server (see Section 12.6) or has otherwise
improperly encoded the stream (e.g., by violating the rules of the
[UTF-8] encoding).
(In the following example, the client attempts to encode data using
UTF-16 instead of UTF-8.)
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C: <?xml version='1.0' encoding='UTF-16'?>
<stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <?xml version='1.0'?>
<stream:stream
from='im.example.com'
id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
<stream:error>
<unsupported-encoding
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.23. unsupported-feature
The receiving entity has advertised a mandatory stream feature that
the initiating entity does not support, and has offered no other
mandatory feature alongside the unsupported feature.
(In the following example, the receiving entity requires negotiation
of an example feature but the initiating entity does not support the
feature.)
R: <stream:features>
<example xmlns='urn:xmpp:example'/>
</stream:features>
I: <stream:error>
<unsupported-feature
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.24. unsupported-stanza-type
The initiating entity has sent a first-level child of the stream that
is not supported by the server or consistent with the default
namespace.
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(In the following example, the client attempts to send an XML stanza
of <pubsub/> when the default namespace is "jabber:client".)
C: <pubsub>
<publish node='princely_musings'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
<entry xmlns='http://www.w3.org/2005/Atom'>
<title>Soliloquy</title>
<summary>
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
</summary>
<link rel='alternate' type='text/html'
href='http://denmark.example/2003/12/13/atom03'/>
<id>tag:denmark.example,2003:entry-32397</id>
<published>2003-12-13T18:30:02Z</published>
<updated>2003-12-13T18:30:02Z</updated>
</entry>
</item>
</publish>
</pubsub>
S: <stream:error>
<unsupported-stanza-type
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>
5.6.3.25. unsupported-version
The value of the 'version' attribute provided by the initiating
entity in the stream header specifies a version of XMPP that is not
supported by the server; the server MAY specify the version(s) it
supports in the <text/> element.
(In the following example, the client specifies an XMPP version of
"11.0" but the server supports only version "1.0" and "1.1".)
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XMPP includes a method for securing the stream from tampering and
eavesdropping. This channel encryption method makes use of the
Transport Layer Security [TLS] protocol, specifically a "STARTTLS"
extension that is modelled after similar extensions for the [IMAP],
[POP3], and [ACAP] protocols as described in [USINGTLS]. The XML
namespace name for the STARTTLS extension is
'urn:ietf:params:xml:ns:xmpp-tls'.
Support for STARTTLS is REQUIRED in XMPP client and server
implementations. An administrator of a given deployment MAY
necessitate the use of TLS for client-to-server communication,
server-to-server communication, or both. A deployed client SHOULD
use TLS to secure its stream with a server prior to attempting the
completion of SASL negotiation (Section 7), and deployed servers
SHOULD use TLS between two domains for the purpose of securing
server-to-server communication.
6.2. Stream Negotiation Rules6.2.1. Mandatory-to-Negotiate
If the receiving entity advertises only the STARTTLS feature or if
the receiving entity includes the <required/> child element, the
parties MUST consider TLS as mandatory-to-negotiate. If TLS is
mandatory-to-negotiate, the receiving entity SHOULD NOT advertise
support for any stream feature except STARTTLS during the initial
stage of the stream negotiation process, because further stream
features might depend on prior negotiation of TLS given the order of
layers in XMPP (e.g., the particular SASL mechanisms offered by the
receiving entity will likely depend on whether TLS has been
negotiated).
6.2.2. Restart
After TLS negotiation, the parties MUST restart the stream.
6.2.3. Data Formatting
During STARTTLS negotiation, the entities MUST NOT send any
whitespace as separators between XML elements (i.e., from the last
character of the <starttls/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace at depth=1 of the stream
as sent by the initiating entity until the last character of the
<proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace at depth=1 of the stream as sent by the receiving entity).
This prohibition helps to ensure proper security layer byte
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precision. Any such whitespace shown in the STARTTLS examples
provided in this document is included only for the sake of
readability.
6.2.4. Order of Negotiation
If the initiating entity chooses to use TLS, STARTTLS negotiation
MUST be completed before proceeding to SASL negotiation (Section 7);
this order of negotiation is necessary to help safeguard
authentication information sent during SASL negotiation, as well as
to make it possible to base the use of the SASL EXTERNAL mechanism on
a certificate (or other credentials) provided during prior TLS
negotiation.
6.3. Process6.3.1. Exchange of Stream Headers and Stream Features
The initiating entity resolves the hostname of the receiving entity
as specified under Section 4, opens a TCP connection to the
advertised port at the resolved IP address, and sends an initial
stream header to the receiving entity; if the initiating entity is
capable of STARTTLS negotiation, it MUST include the 'version'
attribute set to a value of at least "1.0" in the initial stream
header.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST send a response stream header to the
initiating entity over the TCP connection opened by the initiating
entity; if the receiving entity is capable of STARTTLS negotiation,
it MUST include the 'version' attribute set to a value of at least
"1.0" in the response stream header.
R: <stream:stream
from='im.example.com'
id='t7AMCin9zjMNwQKDnplntZPIDEI='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
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The receiving entity then MUST send stream features to the initiating
entity. If the receiving entity supports TLS, the stream features
MUST include an advertisement for support of STARTTLS negotiation,
i.e., a <starttls/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace.
If the receiving entity considers STARTTLS negotiation to be
mandatory, the <starttls/> element SHOULD contain an empty
<required/> child element.
R: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
6.3.2. Initiation of STARTTLS Negotiation6.3.2.1. STARTTLS Command
In order to begin the STARTTLS negotiation, the initiating entity
issues the STARTTLS command (i.e., a <starttls/> element qualified by
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
receiving entity that it wishes to begin a STARTTLS negotiation to
secure the stream.
I: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
The receiving entity MUST reply with either a <proceed/> element
(proceed case) or a <failure/> element (failure case) qualified by
the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.
6.3.2.2. Failure Case
If the failure case occurs, the receiving entity MUST return a
<failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace, terminate the XML stream, and terminate the underlying TCP
connection.
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
R: </stream:stream>
Causes for the failure case include but are not limited to:
1. The initiating entity has sent a malformed STARTTLS command.
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2. The receiving entity does not offer STARTTLS negotiation either
temporarily (e.g., at this stage in the stream negotiation
process) or permanently (i.e., the feature is not supported).
3. The receiving entity cannot complete STARTTLS negotiation because
of an internal error.
Note: STARTTLS failure is not triggered by TLS errors such as bad
certificate or unknown certificate authority; those errors are
generated and handled during the TLS negotiation itself as
described in [TLS].
If the failure case occurs, the initiating entity MAY attempt to
reconnect as explained under Section 4.5.
6.3.2.3. Proceed Case
If the proceed case occurs, the receiving entity MUST return a
<proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
namespace.
R: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
The receiving entity MUST consider the TLS negotiation to have begun
immediately after sending the closing '>' character of the <proceed/>
element to the initiating entity. The initiating entity MUST
consider the TLS negotiation to have begun immediately after
receiving the closing '>' character of the <proceed/> element from
the receiving entity.
The entities now proceed to TLS negotiation as explained in the next
section.
6.3.3. TLS Negotiation6.3.3.1. Rules
In order to complete TLS negotiation over the TCP connection, the
entities MUST follow the process defined in [TLS].
The following rules apply:
1. The entities MUST NOT send any further XML data until the TLS
negotiation has either failed or succeeded.
2. The receiving entity MUST present a certificate.
3. The receiving entity SHOULD send a certificate request to the
initiating entity so that mutual authentication will be possible.
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4. The initiating entity MUST validate the certificate to determine
if the TLS negotiation will succeed; see Section 14.2.2 regarding
certificate validation procedures.
5. The receiving entity SHOULD choose which certificate to present
based on the 'to' attribute of the initial stream header.
Note: See Section 14.6 regarding ciphers that MUST be supported
for TLS; naturally, other ciphers MAY be supported as well.
6.3.3.2. TLS Failure
If the TLS negotiation results in failure, the receiving entity MUST
terminate the TCP connection.
The receiving entity MUST NOT send a closing </stream> tag before
terminating the TCP connection, since the receiving entity and
initiating entity MUST consider the original stream to be replaced
upon failure of the TLS negotiation.
If the failure case occurs and TLS negotation was voluntary
(advertised by means of the <optional/> child element) instead of
mandatory (advertised by means of the <required/> child element), the
initiating entity SHOULD attempt to reconnect as explained under
Section 4.5 but without attempting TLS negotiation.
6.3.3.3. TLS Success
If the TLS negotiation is successful, then the entities MUST proceed
as follows.
1. The initiating entity MUST discard any information transmitted in
layers above TCP that it obtained from the receiving entity in an
insecure manner before TLS took effect (e.g., the receiving
entity's from address or the stream ID and stream features
received from the receiving entity).
2. The receiving entity MUST discard any information transmitted in
layers above TCP that it obtained from the initiating entity in
an insecure manner TLS took effect (e.g., the initiating entity's
from address).
3. The initiating entity MUST send a new initial stream header to
the receiving entity over the encrypted connection.
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I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Note: The initiating entity MUST NOT send a closing </stream> tag
before sending the new initial stream header, since the receiving
entity and initiating entity MUST consider the original stream to
be replaced upon success of the TLS negotiation.
4. The receiving entity MUST respond with a new response stream
header over the encrypted connection.
R: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
5. The receiving entity also MUST send stream features to the
initiating entity, which MUST NOT include the STARTTLS feature
but which SHOULD include the SASL stream feature as described
under Section 7.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
7. SASL Negotiation7.1. Overview
XMPP includes a method for authenticating a stream by means of an
XMPP-specific profile of the Simple Authentication and Security Layer
protocol (see [SASL]). SASL provides a generalized method for adding
authentication support to connection-based protocols, and XMPP uses
an XML namespace profile of SASL that conforms to the profiling
requirements of [SASL].
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Support for SASL negotiation is REQUIRED in XMPP client and server
implementations.
7.2. Stream Negotiation Rules7.2.1. Mandatory-to-Negotiate
The parties to a stream MUST consider SASL as mandatory-to-negotiate.
7.2.2. Restart
After SASL negotiation, the parties MUST restart the stream.
7.2.3. Mechanism Preferences
Any entity that will act as a SASL client or a SASL server MUST
maintain an ordered list of its preferred SASL mechanisms according
to the client or server, where the list is ordered by the perceived
strength of the mechanisms. A server MUST offer and a client MUST
try SASL mechanisms in the order of their perceived strength. For
example, if the server offers the ordered list "PLAIN DIGEST-MD5
GSSAPI" or "DIGEST-MD5 GSSAPI PLAIN" but the client's ordered list is
"GSSAPI DIGEST-MD5", the client MUST try GSSAPI first and then
DIGEST-MD5 but MUST never try PLAIN (since PLAIN is not on its list).
7.2.4. Mechanism Offers
If the receiving entity considers TLS negotiation (Section 6) to be
mandatory before use of a particular SASL authentication mechanism
will be acceptable, the receiving entity MUST NOT advertise that
mechanism in its list of available SASL authentication mechanisms
prior to successful TLS negotiation.
If during prior TLS negotiation the initiating entity presented a
certificate that is acceptable to the receiving entity for purposes
of strong identity verification in accordance with local service
policies, the receiving entity MUST offer the SASL EXTERNAL mechanism
to the initiating entity during SASL negotiation (refer to [SASL])
and SHOULD prefer that mechanism. However, the EXTERNAL mechanism
MAY be offered under other circumstances as well.
See Section 14.6 regarding mechanisms that MUST be supported;
naturally, other SASL mechanisms MAY be supported as well. Best
practices for the use of several SASL mechanisms in the context of
XMPP are described in [XEP-0175] and [XEP-0178].
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The following data formattting rules apply to the SASL negotiation:
1. During SASL negotiation, the entities MUST NOT send any
whitespace as separators between XML elements (i.e., from the
last character of the <auth/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the
stream as sent by the initiating entity until the last character
of the <success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the
stream as sent by the receiving entity). This prohibition helps
to ensure proper security layer byte precision. Any such
whitespace shown in the SASL examples provided in this document
is included only for the sake of readability.
2. Any XML character data contained within the XML elements MUST be
encoded using base64, where the encoding adheres to the
definition in Section 4 of [BASE64] and where the padding bits
are set to zero.
3. As formally specified in the XML schema for the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace under Appendix C.4,
the receiving entity MAY include one or more application-specific
child elements inside the <mechanisms/> element to provide
information that might be needed by the initiating entity in
order to complete successful SASL negotiation using one or more
of the offered mechanisms; however, the syntax and semantics of
all such elements are out of scope for this specification.
7.2.6. Security Layers
Upon successful SASL negotiation that involves negotiation of a
security layer, both the initiating entity and the receiving MUST
discard any application-layer state (i.e, state from the XMPP layer,
excluding state from the TLS negotiation or SASL negotiation).
7.2.7. Simple Username
Some SASL mechanisms have the concept of a "simple username" (e.g.,
CRAM-MD5, DIGEST-MD5, and SCRAM). The exact form of the simple
username in any particular mechanism is a local matter, and a simple
username does not necessarily map to an application identifier (e.g.,
a JID or JID component). Although it is reasonable for an XMPP
application to default the simple username to an XMPP domainpart (for
server-to-server authentication) or to a bare JID (for client-to-
server authentication), an application SHOULD provide a means for a
human user to configure the simple username (as well as other aspects
of credentials, such as realms) in cases where the default is
unsuitable.
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An authorization identity is an optional identity specified by the
initiating entity, which is typically used by an administrator to
perform some management task on behalf of another user. If the
initiating entity wishes to act on behalf of another entity and the
selected SASL mechanism supports transmission of an authorization
identity, the initiating entity MUST provide an authorization
identity during SASL negotiation. If the initiating entity does not
wish to act on behalf of another entity, it MUST NOT provide an
authorization identity. As specified in [SASL], the initiating
entity MUST NOT provide an authorization identity unless the
authorization identity is different from the default authorization
identity derived from the authentication identity. If provided, the
value of the authorization identity MUST be a bare JID of the form
<domain> (i.e., an XMPP domainpart only) for servers and a bare JID
of the form <localpart@domain> (i.e., localpart and domainpart) for
clients.
Note: The authorization identity communicated during SASL
negotiation is used to determine the canonical address for the
initiating client or server according to the receiving server, as
described under Section 3.5.
7.2.9. Realms
The receiving entity MAY include a realm when negotiating certain
SASL mechanisms. If the receiving entity does not communicate a
realm, the initiating entity MUST NOT assume that any realm exists.
The realm MUST be used only for the purpose of authentication; in
particular, an initiating entity MUST NOT attempt to derive an XMPP
hostname from the realm information provided by the receiving entity.
7.2.10. Round Trips
[SASL] specifies that a using protocol such as XMPP can define two
methods by which the protocol can save round trips where allowed for
the SASL mechanism:
1. When the SASL client (the XMPP "initiating entity") requests an
authentication exchange, it can include "initial response" data
with its request if appropriate for the SASL mechanism in use.
In XMPP this is done by including the initial response as the XML
character data of the <auth/> element.
2. At the end of the authentication exchange, the SASL server (the
XMPP "receiving entity") can include "additional data with
success" if appropriate for the SASL mechanism in use. In XMPP
this is done by including the additional data as the XML
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character data of the <success/> element.
For the sake of protocol efficiency, it is RECOMMENDED for XMPP
clients and servers to use these methods, however they MUST support
the less efficient modes as well.
7.3. Process
The process for SASL negotiation is as follows.
7.3.1. Exchange of Stream Headers and Stream Features
If SASL negotiation follows successful STARTTLS negotation
(Section 6), then the SASL negotiation occurs over the encrypted
stream that has already been negotiated. If not, the initiating
entity resolves the hostname of the receiving entity as specified
under Section 4, opens a TCP connection to the advertised port at the
resolved IP address, and sends an initial stream header to the
receiving entity; if the initiating entity is capable of STARTTLS
negotiation, it MUST include the 'version' attribute set to a value
of at least "1.0" in the initial stream header.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST send a response stream header to the
initiating entity; if the receiving entity is capable of SASL
negotiation, it MUST include the 'version' attribute set to a value
of at least "1.0" in the response stream header.
R: <stream:stream
from='im.example.com'
id='vgKi/bkYME8OAj4rlXMkpucAqe4='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
The receiving entity also MUST send stream features to the initiating
entity. If the receiving entity supports SASL, the stream features
MUST include an advertisement for support of SASL negotiation, i.e.,
a <mechanisms/> element qualified by the
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'urn:ietf:params:xml:ns:xmpp-sasl' namespace.
The <mechanisms/> element MUST contain one <mechanism/> child element
for each authentication mechanism the receiving entity offers to the
initiating entity. The order of <mechanism/> elements in the XML
indicates the preference order of the SASL mechanisms according to
the receiving entity; however the initiating entity MUST maintain its
own preference order independent of the preference order of the
receiving entity.
R: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
7.3.2. Initiation
In order to begin the SASL negotiation, the initiating entity sends
an <auth/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an
appropriate value for the 'mechanism' attribute. This element MAY
contain XML character data (in SASL terminology, the "initial
response") if the mechanism supports or requires it; if the
initiating entity needs to send a zero-length initial response, it
MUST transmit the response as a single equals sign character ("="),
which indicates that the response is present but contains no data.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
7.3.3. Challenge-Response Sequence
If necessary, the receiving entity challenges the initiating entity
by sending a <challenge/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
The initiating entity responds to the challenge by sending a
<response/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (which MUST be generated in accordance
with the definition of the SASL mechanism chosen by the initiating
entity).
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If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
o The initiating entity aborts the handshake.
o The receiving entity reports failure of the handshake.
o The receiving entity reports success of the handshake.
These scenarios are described in the following sections.
7.3.4. Abort
The initiating entity aborts the handshake by sending an <abort/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Upon receiving an <abort/> element, the receiving entity MUST return
a <failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an
<aborted/> child element.
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
<aborted/>
</failure>
7.3.5. Failure
The receiving entity reports failure of the handshake by sending a
<failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of
failure MUST be communicated in an appropriate child element of the
<failure/> element as defined under Section 7.4).
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
Where appropriate for the chosen SASL mechanism, the receiving entity
SHOULD allow a configurable but reasonable number of retries (at
least 2 and no more than 5); this enables the initiating entity
(e.g., an end-user client) to tolerate incorrectly-provided
credentials (e.g., a mistyped password) without being forced to
reconnect.
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If the initiating entity attempts a reasonable number of retries with
the same SASL mechanism and all attempts fail, it MAY fall back to
the next mechanism in its ordered list by sending a new <auth/>
request to the receiving entity. If there are no remaining
mechanisms in its list, the initiating entity SHOULD instead send an
<abort/> element to the receiving entity.
If the initiating entity exceeds the number of retries, the receiving
entity MUST return a stream error (which SHOULD be <policy-
violation/> but MAY be <not-authorized/>).
7.3.6. Success
The receiving entity reports success of the handshake by sending a
<success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
contain XML character data (in SASL terminology, "additional data
with success") if the chosen SASL mechanism supports or requires it;
if the receiving entity needs to send additional data of zero length,
it MUST transmit the data as a single equals sign character ("=").
R: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Note: The authorization identity communited during SASL
negotiation is used to determine the canonical address for the
initiating client or server according to the receiving server, as
described under Section 3.5.
Upon receiving the <success/> element, the initiating entity MUST
initiate a new stream over the existing TCP connection by sending a
new initial stream header to the receiving entity.
I: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
Note: The initiating entity MUST NOT send a closing </stream> tag
before sending the new initial stream header, since the receiving
entity and initiating entity MUST consider the original stream to
be replaced upon sending or receiving the <success/> element.
Upon receiving the new initial stream header from the initiating
entity, the receiving entity MUST respond by sending a new response
XML stream header to the initiating entity.
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R: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
The receiving entity MUST also send stream features, containing any
further available features or containing no features (via an empty
<features/> element).
R: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
7.4. SASL Errors
The syntax of SASL errors is as follows:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<defined-condition/>
[<text xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
</failure>
Where "defined-condition" is one of the SASL-related error conditions
defined in the following sections.
Inclusion of a defined condition is REQUIRED.
Inclusion of the <text/> element is OPTIONAL, and can be used to
provide application-specific information about the error condition,
which information MAY be displayed to a human but only as a
supplement to the defined condition.
7.4.1. aborted
The receiving entity acknowledges an <abort/> element sent by the
initiating entity; sent in reply to the <abort/> element.
I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<aborted/>
</failure>
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The account of the initiating entity has been temporarily disabled;
sent in reply to an <auth/> element (with or without initial response
data) or a <response/> element.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<account-disabled/>
<text xml:lang='en'>Call 212-555-1212 for assistance.</text>
</failure>
7.4.3. credentials-expired
The authentication failed because the initiating entity provided
credentials that have expired; sent in reply to a <response/> element
or an <auth/> element with initial response data.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<credentials-expired/>
</failure>
7.4.4. encryption-required
The mechanism requested by the initiating entity cannot be used
unless the underlying stream is encrypted; sent in reply to an
<auth/> element (with or without initial response data).
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<encryption-required/>
</failure>
7.4.5. incorrect-encoding
The data provided by the initiating entity could not be processed
because the [BASE64] encoding is incorrect (e.g., because the
encoding does not adhere to the definition in Section 4 of [BASE64]);
sent in reply to a <response/> element or an <auth/> element with
initial response data.
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I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'>[ ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
7.4.6. invalid-authzid
The authzid provided by the initiating entity is invalid, either
because it is incorrectly formatted or because the initiating entity
does not have permissions to authorize that ID; sent in reply to a
<response/> element or an <auth/> element with initial response data.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-authzid/>
</failure>
7.4.7. invalid-mechanism
The initiating entity did not provide a mechanism or requested a
mechanism that is not supported by the receiving entity; sent in
reply to an <auth/> element.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='CRAM-MD5'/>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-mechanism/>
</failure>
7.4.8. malformed-request
The request is malformed (e.g., the <auth/> element includes initial
response data but the mechanism does not allow that, or the data sent
violates the syntax for the specified SASL mechanism); sent in reply
to an <abort/>, <auth/>, <challenge/>, or <response/> element.
(In the following example, the XML character data of the <auth/>
element contains more than 255 UTF-8-encoded Unicode characters and
therefore violates the "token" production for the SASL ANONYMOUS
mechanism as specified in [ANONYMOUS].)
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I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='ANONYMOUS'>[ ... some-long-token ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<malformed-request/>
</failure>
7.4.9. mechanism-too-weak
The mechanism requested by the initiating entity is weaker than
server policy permits for that initiating entity; sent in reply to an
<auth/> element (with or without initial response data).
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism-too-weak/>
</failure>
7.4.10. not-authorized
The authentication failed because the initiating entity did not
provide proper credentials or the receiving entity has detected an
attack but wishes to disclose as little information as possible to
the attacker; sent in reply to a <response/> element or an <auth/>
element with initial response data.
I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
Note: This error condition includes but is not limited to the case
of incorrect credentials or an unknown username. In order to
discourage directory harvest attacks, no differentiation is made
between incorrect credentials and an unknown username.
7.4.11. temporary-auth-failure
The authentication failed because of a temporary error condition
within the receiving entity, and it is advisable for the initiating
entity to try again later; sent in reply to an <auth/> element or a
<response/> element.
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I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
[ ... ]
</response>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
7.4.12. transition-needed
The authentication failed because the mechanism cannot be used until
the initiating entity provides (for one time only) a plaintext
password so that the receiving entity can build a hashed password for
use in future authentication attempts; sent in reply to an <auth/>
element with or without initial response data.
I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='CRAM-MD5'>[ ... ]</auth>
R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<transition-needed/>
</failure>
Note: An XMPP client MUST treat a <transition-needed/> SASL error
with extreme caution, SHOULD NOT provide a plaintext password over
an XML stream that is not encrypted via Transport Layer Security,
and MUST warn a human user before allowing the user to provide a
plaintext password over an unencrypted connection.
7.5. SASL Definition
The profiling requirements of [SASL] require that the following
information be supplied by the definition of a using protocol.
service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possessed by an <auth/> element, optionally
including an initial response to avoid a round trip.
exchange sequence: Challenges and responses are carried through the
exchange of <challenge/> elements from receiving entity to
initiating entity and <response/> elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a <failure/> element and success by sending a <success/>
element; the initiating entity aborts the exchange by sending an
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<abort/> element. Upon successful negotiation, both sides
consider the original XML stream to be closed and new stream
headers are sent by both entities.
security layer negotiation: The security layer takes effect
immediately after sending the closing '>' character of the
<success/> element for the receiving entity, and immediately after
receiving the closing '>' character of the <success/> element for
the initiating entity. The order of layers is first [TCP], then
[TLS], then [SASL], then XMPP.
use of the authorization identity: The authorization identity can be
used in XMPP to denote the non-default <localpart@domain> of a
client or the sending <domain> of a server; an empty string is
equivalent to an absent authorization identity.
8. Resource Binding8.1. Overview
After a client authenticates with a server, it MUST bind a specific
resource to the stream so that the server can properly address the
client (see Section 3). That is, there MUST be an XMPP resource
associated with the bare JID (<localpart@domain>) of the client, so
that the address for use over that stream is a full JID of the form
<localpart@domain/resource> (including the resourcepart). This
ensures that the server can deliver XML stanzas to and receive XML
stanzas from the client in relation to entities other than the server
itself, as explained under Section 11 (the client could exchange
stanzas with the server itself before binding a resource since the
full JID is needed only for addressing outside the context of the
stream negotiated between the client and the server, but this is not
commonly done).
After a client has bound a resource to the stream, it is referred to
as a CONNECTED RESOURCE. A server SHOULD allow an entity to maintain
multiple connected resources simultaneously, where each connected
resource is associated with a distinct XML stream and differentiated
from the other connected resources by a distinct resourcepart;
however, a server MUST enable the administrator of an XMPP service to
limit the number of connected resources in order to prevent certain
denial of service attacks as described under Section 14.14.
If, before completing the resource binding step, the client attempts
to send an XML stanza to an entity other than the client's account or
the server, the server MUST NOT process the stanza and MUST either
ignore the stanza or return a <not-authorized/> stream error to the
client.
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Support for resource binding is REQUIRED in XMPP client and server
implementations.
8.2. Stream Negotiation Rules8.2.1. Mandatory-to-Negotiate
The parties to a stream MUST consider resource binding as mandatory-
to-negotiate.
8.2.2. Restart
After resource binding, the parties MUST NOT restart the stream.
8.3. Advertising Support
Upon sending a new response stream header to the client after
successful SASL negotiation, the server MUST include a <bind/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace
in the stream features it presents to the client.
Note: Resource binding is mandatory-to-negotiate for clients.
Note: The server MUST NOT include the resource binding stream
feature until after the client has authenticated, typically by
means of successful SASL negotiation.
S: <stream:stream
from='im.example.com'
id='gPybzaOzBmaADgxKXu9UClbprp0='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
S: <stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
Upon being so informed that resource binding is mandatory, the client
MUST bind a resource to the stream as described in the following
sections.
8.4. Generation of Resource Identifiers
A resourcepart MUST at a minimum be unique among the connected
resources for that <localpart@domain>. Enforcement of this policy is
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the responsibility of the server.
A resourcepart can be security-critical. For example, if a malicious
entity can guess a client's resourcepart then it might be able to
determine if the client (and therefore the controlling principal) is
online or offline, thus resulting in a presence leak as described
under Section 14.15. To prevent that possibility, a client can
either (1) generate a random resourcepart on its own or (2) ask the
server to generate a resourcepart on its behalf, which MUST be random
(see [RANDOM]). When generating a random resourcepart, it is
RECOMMENDED that the resourcepart be a Universally Unique Identifier
(UUID), for which the format specified in [UUID] is RECOMMENDED.
8.5. Server-Generated Resource Identifier
A server that supports resource binding MUST be able to generate an
XMPP resourcepart on behalf of a client.
8.5.1. Success Case
A client requests a server-generated resourcepart by sending an IQ
stanza of type "set" (see Section 9.2.3) containing an empty <bind/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
namespace.
C: <iq id='bind_1' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>
Once the server has generated an XMPP resourcepart for the client, it
MUST return an IQ stanza of type "result" to the client, which MUST
include a <jid/> child element that specifies the full JID for the
connected resource as determined by the server.
S: <iq id='bind_1' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>
8.5.2. Error Cases
When a client asks the server to generate a resourcepart during
resource binding, the following stanza error conditions are possible:
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o The request is malformed.
o The account has reached a limit on the number of simultaneous
connected resources allowed.
o The client is otherwise not allowed to bind a resource to the
stream.
8.5.2.1. Resource Constraint
If the account has reached a limit on the number of simultaneous
connected resources allowed, the server MUST return a <resource-
constraint/> stanza error.
S: <iq id='bind_2' type='error'>
<error type='wait'>
<resource-constraint
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.5.2.2. Not Allowed
If the client is otherwise not allowed to bind a resource to the
stream, the server MUST return a <not-allowed/> stanza error.
S: <iq id='bind_2' type='error'>
<error type='cancel'>
<not-allowed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.6. Client-Submitted Resource Identifier
Instead of asking the server to generate a resourcepart on its
behalf, a client MAY attempt to submit a resourcepart that it has
generated or that the controlling user has provided.
8.6.1. Success Case
A client asks its server to accept a client-submitted resourcepart by
sending an IQ stanza of type "set" containing a <bind/> element with
a child <resource/> element containing non-zero-length XML character
data.
C: <iq id='bind_2' type='set'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>balcony</resource>
</bind>
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</iq>
The server SHOULD accept the client-submitted resourcepart. It does
so by returning an IQ stanza of type "result" to the client,
including a <jid/> child element that specifies the full JID for the
connected resource and contains without modification the client-
submitted text.
S: <iq id='bind_2' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>juliet@im.example.com/balcony</jid>
</bind>
</iq>
8.6.2. Error Cases
When a client attempts to submit its own XMPP resourcepart during
resource binding, the following stanza error conditions are possible
in addition to those described under Section 8.5.2:
o The provided resourcepart cannot be processed by the server.
o The provided resourcepart is already in use.
8.6.2.1. Bad Request
If the provided resourcepart cannot be processed by the server (e.g.
because it is of zero length or because it is not in accordance with
the Resourceprep (Appendix B) profile of [STRINGPREP]), the server
MAY return a <bad-request/> stanza error (but SHOULD instead apply
the Resourceprep (Appendix B) profile of [STRINGPREP] or otherwise
process the resourcepart so that it is in conformance).
S: <iq id='bind_2' type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.6.2.2. Conflict
If there is already a connected resource of the same name, the server
MUST do one of the following:
1. Not accept the resourcepart provided by the client but instead
override it with an XMPP resourcepart that the server generates.
2. Terminate the current resource and allow the newly-requested
resource.
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3. Disallow the newly-requested resource and maintain the current
resource.
Which of these the server does is up to the implementation, although
it is RECOMMENDED to implement case #1.
S: <iq id='bind_2' type='result'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>
juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
</jid>
</bind>
</iq>
In case #2, the server MUST send a <conflict/> stream error to the
current resource and return an IQ stanza of type "result" (indicating
success) to the newly-requested resource.
S: <iq id='bind_2' type='result'/>
In case #3, the server MUST send a <conflict/> stanza error to the
newly-requested resource but maintain the XML stream for that
connection so that the newly-requested resource has an opportunity to
negotiate a non-conflicting resourcepart before sending another
request for resource binding.
S: <iq id='bind_2' type='error'>
<error type='modify'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8.6.3. Retries
If an error occurs when a client submits a resourcepart, the server
SHOULD allow a configurable but reasonable number of retries (at
least 2 and no more than 5); this enables the client to tolerate
incorrectly-provided resourceparts (e.g., bad data formats or
duplicate text strings) without being forced to reconnect.
After the client has reached the retry limit, the server MUST return
a <policy-violation/> stream error to the client.
9. XML Stanzas
After a client has connected to a server or two servers have
connected to each other, either party can send XML stanzas over the
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negotiated stream. Three kinds of XML stanza are defined for the
'jabber:client' and 'jabber:server' namespaces: <message/>,
<presence/>, and <iq/>. In addition, there are five common
attributes for these stanza types. These common attributes, as well
as the basic semantics of the three stanza types, are defined herein;
more detailed information regarding the syntax of XML stanzas for
instant messaging and presence applications is provided in [XMPP-IM],
and for other applications in the relevant XMPP extension
specifications.
A server MUST NOT process a partial stanza and MUST NOT attach
meaning to the transmission timing of any part of a stanza (before
receipt of the close tag).
Support for the XML stanza syntax and semantics defined herein is
REQUIRED in XMPP client and server implementations.
9.1. Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas.
9.1.1. to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
<message to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
For information about server processing of inbound and outbound XML
stanzas based on the nature of the 'to' address, refer to Section 11.
9.1.1.1. Client-to-Server Streams
The following rules apply to inclusion of the 'to' attribute in the
context of XML streams qualified by the 'jabber:client' namespace
(i.e., client-to-server streams).
1. A stanza with a specific intended recipient MUST possess a 'to'
attribute whose value is an XMPP address.
2. A stanza sent from a client to a server for direct processing by
the server on behalf of the client (e.g., presence sent to the
server for broadcasting to other entities) MUST NOT possess a
'to' attribute.
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The following rules apply to inclusion of the 'to' attribute in the
context of XML streams qualified by the 'jabber:server' namespace
(i.e., server-to-server streams).
1. A stanza MUST possess a 'to' attribute whose value is an XMPP
address; if a server receives a stanza that does not meet this
restriction, it MUST generate an <improper-addressing/> stream
error.
2. The domainpart of the JID in the 'to' atttribute MUST match a
hostname serviced by the receiving server; if a server receives a
stanza that does not meet this restriction, it MUST generate a
<host-unknown/> or <host-gone/> stream error.
9.1.2. from
The 'from' attribute specifies the JID of the sender.
<message from='juliet@im.example.com/balcony'
to='romeo@example.net'>
<body>Art thou not Romeo, and a Montague?</body>
</message>
9.1.2.1. Client-to-Server Streams
The following rules apply to the 'from' attribute in the context of
XML streams qualified by the 'jabber:client' namespace (i.e., client-
to-server streams).
1. When the server receives an XML stanza from a client and the
stanza does not include a 'from' attribute, the server MUST add a
'from' attribute to the stanza, where the value of the 'from'
attribute is the full JID (<localpart@domain/resource>)
determined by the server for the connected resource that
generated the stanza (see Section 3.5), or the bare JID
(<localpart@domain>) in the case of subscription-related presence
stanzas (see [XMPP-IM]).
2. When the server receives an XML stanza from a client and the
stanza includes a 'from' attribute, the server MUST either (a)
validate that the value of the 'from' attribute provided by the
client is that of a connected resource for the associated entity
or (b) override the provided 'from' attribute by adding a 'from'
attribute as specified under Rule #1.
3. When the server generates a stanza from the server for delivery
to the client on behalf of the account of the connected client
(e.g., in the context of data storage services provided by the
server on behalf of the client), the stanza MUST either (a) not
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include a 'from' attribute or (b) include a 'from' attribute
whose value is the account's bare JID (<localpart@domain>).
4. When the server generates a stanza from the server itself for
delivery to the client, the stanza MUST include a 'from'
attribute whose value is the bare JID (i.e., <domain>) of the
server.
5. A server MUST NOT send to the client a stanza without a 'from'
attribute if the stanza was not generated by the server (e.g., if
it was generated by another client or another server); therefore,
when a client receives a stanza that does not include a 'from'
attribute, it MUST assume that the stanza is from the server to
which the client is connected.
9.1.2.2. Server-to-Server Streams
The following rules apply to the 'from' attribute in the context of
XML streams qualified by the 'jabber:server' namespace (i.e., server-
to-server streams).
1. A stanza MUST possess a 'from' attribute whose value is an XMPP
address; if a server receives a stanza that does not meet this
restriction, it MUST generate an <improper-addressing/> stream
error.
2. The domainpart of the JID contained in the 'from' attribute MUST
match the hostname of the sending server (or any validated domain
thereof) as communicated in the SASL negotiation (see Section 7),
server dialback (see [XEP-0220], or similar means; if a server
receives a stanza that does not meet this restriction, it MUST
generate an <invalid-from/> stream error.
Enforcement of these rules helps to prevent certain denial of service
attacks as described under Section 14.14.
9.1.3. id
The 'id' attribute is used by the entity that generates a stanza
("the originating entity") to track any response or error stanza that
it might receive in relation to the generated stanza from another
entity (such as an intermediate server or the intended recipient).
It is up to the originating entity whether the value of the 'id'
attribute will be unique only within its current stream (session) or
unique globally.
For <message/> and <presence/> stanzas, it is RECOMMENDED for the
originating entity to include an 'id' attribute; for <iq/> stanzas,
it is REQUIRED.
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If the generated stanza includes an 'id' attribute then it is
REQUIRED for the response or error stanza to also include an 'id'
attribute, where the value of the 'id' attribute MUST match that of
the generated stanza.
Note: The semantics of IQ stanzas impose additional restrictions; see
Section 9.2.3.
9.1.4. type
The 'type' attribute specifies the purpose or context of the message,
presence, or IQ stanza. The particular allowable values for the
'type' attribute vary depending on whether the stanza is a message,
presence, or IQ stanza. The defined values for message and presence
stanzas are specific to instant messaging and presence applications
and therefore are specified in [XMPP-IM], whereas the values for IQ
stanzas specify the role of an IQ stanza in a structured request-
response exchange and therefore are specified under Section 9.2.3.
The only 'type' value common to all three stanzas is "error"; see
Section 9.3.
9.1.5. xml:lang
A stanza SHOULD possess an 'xml:lang' attribute (as defined in
Section 2.12 of [XML]) if the stanza contains XML character data that
is intended to be presented to a human user (as explained in
[CHARSET], "internationalization is for humans"). The value of the
'xml:lang' attribute specifies the default language of any such
human-readable XML character data.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
The value of the 'xml:lang' attribute MAY be overridden by the 'xml:
lang' attribute of a specific child element.
<presence from='romeo@example.net/orchard' xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
<status xml:lang='cs'>Dvo&#x0159;&#x00ED;m se Julii</status>
</presence
If an outbound stanza generated by a client does not possess an 'xml:
lang' attribute, the client's server SHOULD add an 'xml:lang'
attribute whose value is that specified for the stream as defined
under Section 5.4.4.
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C: <presence from='romeo@example.net/orchard'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
S: <presence from='romeo@example.net/orchard'
to='juliet@im.example.com'
xml:lang='en'>
<show>dnd</show>
<status>Wooing Juliet</status>
</presence>
If an inbound stanza received received by a client or server does not
possess an 'xml:lang' attribute, an implementation MUST assume that
the default language is that specified for the stream as defined
under Section 5.4.4.
The value of the 'xml:lang' attribute MUST conform to the NMTOKEN
datatype (as defined in Section 2.3 of [XML]) and MUST conform to the
format defined in [LANGTAGS].
A server MUST NOT modify or delete 'xml:lang' attributes on stanzas
it receives from other entities.
9.2. Basic Semantics9.2.1. Message Semantics
The <message/> stanza can be seen as a "push" mechanism whereby one
entity pushes information to another entity, similar to the
communications that occur in a system such as email. All message
stanzas SHOULD possess a 'to' attribute that specifies the intended
recipient of the message; upon receiving such a stanza, a server
SHOULD route or deliver it to the intended recipient (see Section 11
for general routing and delivery rules related to XML stanzas).
9.2.2. Presence Semantics
The <presence/> stanza can be seen as a specialized broadcast or
"publish-subscribe" mechanism, whereby multiple entities receive
information (in this case, network availability information) about an
entity to which they have subscribed. In general, a publishing
entity (client) SHOULD send a presence stanza with no 'to' attribute,
in which case the server to which the entity is connected SHOULD
broadcast that stanza to all subscribed entities. However, a
publishing entity MAY also send a presence stanza with a 'to'
attribute, in which case the server SHOULD route or deliver that
stanza to the intended recipient. See Section 11 for general routing
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and delivery rules related to XML stanzas, and [XMPP-IM] for rules
specific to presence applications.
9.2.3. IQ Semantics
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to the Hypertext Transfer Protocol [HTTP]. The semantics of IQ
enable an entity to make a request of, and receive a response from,
another entity. The data content of the request and response is
defined by the schema or other structural definition associated with
the XML namespace that qualifies the direct child element of the IQ
element (see Section 9.4), and the interaction is tracked by the
requesting entity through use of the 'id' attribute. Thus, IQ
interactions follow a common pattern of structured data exchange such
as get/result or set/result (although an error can be returned in
reply to a request if appropriate):
Requesting Responding
Entity Entity
---------- ----------
| |
| <iq id='1' type='get'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='1' type='result'> |
| [ ... payload ... ] |
| </iq> |
| <------------------------- |
| |
| <iq id='2' type='set'> |
| [ ... payload ... ] |
| </iq> |
| -------------------------> |
| |
| <iq id='2' type='error'> |
| [ ... condition ... ] |
| </iq> |
| <------------------------- |
| |
To enforce these semantics, the following rules apply:
1. The 'id' attribute is REQUIRED for IQ stanzas.
2. The 'type' attribute is REQUIRED for IQ stanzas. The value MUST
be one of the following (if the value is other than one of the
following strings, the recipient or an intermediate router MUST
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return a stanza error of <bad-request/>):
* get -- The stanza requests information, inquires about what
data is needed in order to complete further operations, etc.
* set -- The stanza provides data that is needed for an
operation to be completed, sets new values, replaces existing
values, etc.
* result -- The stanza is a response to a successful get or set
request.
* error -- The stanza reports an error that has occurred
regarding processing or delivery of a previously-sent get or
set request (see Section 9.3).
3. An entity that receives an IQ request of type "get" or "set" MUST
reply with an IQ response of type "result" or "error". The
response MUST preserve the 'id' attribute of the request.
4. An entity that receives a stanza of type "result" or "error" MUST
NOT respond to the stanza by sending a further IQ response of
type "result" or "error"; however, the requesting entity MAY send
another request (e.g., an IQ of type "set" to provide obligatory
information discovered through a get/result pair).
5. An IQ stanza of type "get" or "set" MUST contain exactly one
child element, which specifies the semantics of the particular
request.
6. An IQ stanza of type "result" MUST include zero or one child
elements.
7. An IQ stanza of type "error" MAY include the child element
contained in the associated "get" or "set" and MUST include an
<error/> child; for details, see Section 9.3.
9.3. Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (Section 5.6). Unlike stream errors, stanza errors are
recoverable; therefore they do not result in termination of the XML
stream and underlying TCP connection. Instead, the entity that
discovers the error condition returns an ERROR STANZA to the sender,
i.e., a stanza of the same kind (message, presence, or IQ) whose
'type' attribute is set to a value of "error" and which contains an
<error/> child element that specifies the error condition. The
specified error condition provides a hint regarding actions that the
sender can take to remedy the error if possible.
9.3.1. Rules
The following rules apply to stanza errors:
1. The receiving or processing entity that detects an error
condition in relation to a stanza SHOULD return an error stanza
(and MUST do so for IQ stanzas).
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2. The entity that generates an error stanza MAY include the
original XML sent so that the sender can inspect and, if
necessary, correct the XML before attempting to resend.
3. If the generated stanza included an 'id' attribute then it is
REQUIRED for the error stanza to also include an 'id' attribute,
where the value of the 'id' attribute MUST match that of the
generated stanza.
4. An error stanza MUST contain an <error/> child element.
5. An <error/> child MUST NOT be included if the 'type' attribute
has a value other than "error" (or if there is no 'type'
attribute).
6. An entity that receives an error stanza MUST NOT respond to the
stanza with a further error stanza; this helps to prevent
looping.
9.3.2. Syntax
The syntax for stanza-related errors is:
<stanza-kind from='intended-recipient' to='sender' type='error'>
[OPTIONAL to include sender XML here]
<error type='error-type'>
<defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
[<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
xml:lang='langcode'>
OPTIONAL descriptive text
</text>]
[OPTIONAL application-specific condition element]
</error>
</stanza-kind>
The "stanza-kind" MUST be one of message, presence, or iq.
The "error-type" MUST be one of the following:
o auth -- retry after providing credentials
o cancel -- do not retry (the error cannot be remedied)
o continue -- proceed (the condition was only a warning)
o modify -- retry after changing the data sent
o wait -- retry after waiting (the error is temporary)
The <error/> element:
o MUST contain a child element corresponding to one of the stanza
error conditions defined under Section 9.3.3; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.
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o MAY contain a <text/> child element containing XML character data
that describes the error in more detail; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
and SHOULD possess an 'xml:lang' attribute specifying the natural
language of the XML character data.
o MAY contain a child element for an application-specific error
condition; this element MUST be qualified by an application-
specific namespace that defines the syntax and semantics of the
element.
The <text/> element is OPTIONAL. If included, it MUST be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
MUST NOT be interpreted programmatically by an application. It MUST
NOT be used as the error message presented to a human user, but MAY
be shown in addition to the error message associated with the defined
condition element (and, optionally, the application-specific
condition element).
9.3.3. Defined Conditions
The following conditions are defined for use in stanza errors.
9.3.3.1. bad-request
The sender has sent a stanza containing XML that does not conform to
the appropriate schema or that cannot be processed (e.g., an IQ
stanza that includes an unrecognized value of the 'type' attribute,
or an element that is qualified by a recognized namespace but that
violates the defined syntax for the element); the associated error
type SHOULD be "modify".
C: <iq from='juliet@im.example.com/balcony'
id='some-id'
to='im.example.com'
type='subscribe'>
<ping xmlns='urn:xmpp:ping'/>
</iq>
S: <iq from='im.example.com'
id='some-id'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
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The requesting entity does not possess the necessary permissions to
perform the action; the associated error type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='auth'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.5. gone
The recipient or server can no longer be contacted at this address,
typically on a permanent basis; the associated error type SHOULD be
"cancel" or "modify" and the error stanza SHOULD include a new
address as the XML character data of the <gone/> element (which MUST
be a URI or IRI at which the entity can be contacted, typically an
XMPP IRI as specified in [XMPP-URI]).
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
xmpp:conference.example.com
</gone>
</error>
</presence>
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The server could not process the stanza because of a misconfiguration
or an otherwise-undefined internal server error; the associated error
type SHOULD be "wait" or "cancel".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='wait'>
<internal-server-error
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.7. item-not-found
The addressed JID or item requested cannot be found; the associated
error type SHOULD be "cancel" or "modify".
C: <presence from='userfoo@example.com/bar'
to='nosuchroom@conference.example.org/foo'/>
S: <presence from='nosuchroom@conference.example.org/foo'
to='userfoo@example.com/bar'
type='error'>
<error type='cancel'>
<item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
Note: An application MUST NOT return this error if doing so would
provide information about the intended recipient's network
availability to an entity that is not authorized to know such
information; instead it MUST return a <service-unavailable/>
stanza error.
9.3.3.8. jid-malformed
The sending entity has provided or communicated an XMPP address
(e.g., a value of the 'to' attribute) or aspect thereof (e.g., an
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XMPP resourcepart) that does not adhere to the syntax defined under
Section 3; the associated error type SHOULD be "modify".
C: <presence
from='juliet@im.example.com/balcony'
to='ch@r@cters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='ch@r@cters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<jid-malformed
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.9. not-acceptable
The recipient or server understands the request but is refusing to
process it because it does not meet criteria defined by the recipient
or server (e.g., a local policy regarding stanza size limits or
acceptable words in messages); the associated error type SHOULD be
"modify".
C: <message to='juliet@im.example.com' id='foo'>
<body>[ ... the-emacs-manual ... ]</body>
</message>
S: <message from='juliet@im.example.com' id='foo'>
<error type='modify'>
<not-acceptable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
9.3.3.10. not-allowed
The recipient or server does not allow any entity to perform the
action (e.g., sending to entities at a blacklisted domain); the
associated error type SHOULD be "cancel".
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C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='wait'>
<recipient-unavailable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
Note: An application MUST NOT return this error if doing so would
provide information about the intended recipient's network
availability to an entity that is not authorized to know such
information; instead it MUST return a <service-unavailable/>
stanza error.
9.3.3.16. redirect
The recipient or server is redirecting requests for this information
to another entity, typically in a temporary fashion (the <gone/>
condition is used for permanent addressing failures); the associated
error type SHOULD be "modify" and the error stanza SHOULD contain the
alternate address in the XML character data of the <redirect/>
element (which MUST be a URI or IRI at which the entity can be
contacted, typically an XMPP IRI as specified in [XMPP-URI]).
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'
type='error'>
<error type='modify'>
<redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
xmpp:characters@conference.example.org
</redirect>
</error>
</presence>
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The requesting entity is not authorized to access the requested
service because prior registration is necessary; the associated error
type SHOULD be "auth".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='auth'>
<registration-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.18. remote-server-not-found
A remote server or service specified as part or all of the JID of the
intended recipient does not exist; the associated error type SHOULD
be "cancel".
C: <presence
from='juliet@im.example.com/balcony'
to='characters@muc.example.com/JulieC'>
<x xmlns='http://jabber.org/protocol/muc'/>
</presence>
E: <presence
from='characters@muc.example.com/JulieC'
to='juliet@im.example.com/balcony'>
<error type='cancel'>
<remote-server-not-found
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</presence>
9.3.3.19. remote-server-timeout
A remote server or service specified as part or all of the JID of the
intended recipient (or needed to fulfill a request) could not be
contacted within a reasonable amount of time; the associated error
type SHOULD be "wait".
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C: <message from='romeo@example.net/foo'
to='juliet@im.example.com'>
<body>Hello?</body>
</message>
S: <message from='juliet@im.example.com/foo'
to='romeo@example.net'>
<error type='cancel'>
<service-unavailable
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
An application MUST return a <service-unavailable/> stanza error
instead of <item-not-found/> or <recipient-unavailable/> if sending
one of the latter errors would provide information about the intended
recipient's network availability to an entity that is not authorized
to know such information.
9.3.3.22. subscription-required
The requesting entity is not authorized to access the requested
service because a prior subscription is necessary; the associated
error type SHOULD be "auth".
C: <message
from='romeo@example.net/orchard'
to='playbot@shakespeare.example.com'
<body>help</body>
</message>
E: <message
from='playbot@shakespeare.example.com'
to='romeo@example.net/orchard'
type='error'>
<error type='auth'>
<subscription-required
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
9.3.3.23. undefined-condition
The error condition is not one of those defined by the other
conditions in this list; any error type can be associated with this
condition, and it SHOULD be used only in conjunction with an
application-specific condition.
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<message type='error' id='another-id'>
<error type='modify'>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xml:lang='en'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
[ ... application-specific information ... ]
</text>
<too-many-parameters xmlns='http://example.com/ns'/>
</error>
</message>
An entity that receives an application-specific error condition it
does not understand MUST ignore the condition.
9.4. Extended Content
Although the message, presence, and IQ stanzas provide basic
semantics for messaging, availability, and request-response
interactions, XMPP uses XML namespaces (see [XML-NAMES] to extend the
basic stanza syntax for the purpose of providing additional
functionality.
A message or presence stanza MAY contain one or more optional child
elements specifying content that extends the meaning of the message
(e.g., an XHTML-formatted version of the message body as described in
[XEP-0071]), and an IQ stanza of type "get" or "set" MUST contain one
such child element. Such a child element MAY have any name and MUST
possess a namespace declaration (other than "jabber:client", "jabber:
server", or "http://etherx.jabber.org/streams") that defines all data
contained within the child element. Such a child element is called
an "extension element". An extension element can be included either
at the direct child level of the stanza or in any mix of levels.
Similarly, "extension attributes" are allowed. That is: a stanza
itself (i.e., the <iq/>, <message/>, and <presence/> elements
qualified by the "jabber:client" or "jabber:server" namespace
declared as the default namespace for the stream) and any child
element of such a stanza (whether a child element qualifed by the
default namespace or an extension element) MAY also include one or
more attributes that are qualified by XML namespaces that are
different from the default namespace or the reserved "xml" prefix
(including the "empty namespace" if the attribute is not prefixed).
For the sake of backward compatibility and maximum interoperability,
an entity that generates a stanza SHOULD NOT include such attributes
in the stanza itself or in child elements of the stanza that are
qualified by the default namespace (e.g., the message <body/>
element).
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An extension element or extension attribute is said to be EXTENDED
CONTENT and the namespace name for such an element or attribute is
said to be an EXTENDED NAMESPACE.
To illustrate these concepts, several examples follow.
The following stanza contains one direct child element whose extended
namespace is 'jabber:iq:roster':
<iq from='juliet@capulet.com/balcony'
id='roster1'
type='get'>
<query xmlns='jabber:iq:roster'/>
</iq>
The following stanza contains two direct child elements with two
different extended namespaces.
<presence from='juliet@capulet.com/balcony'>
<c xmlns='http://jabber.org/protocol/caps'
node='http://exodus.jabberstudio.org/caps'
ver='0.9'/>
<x xmlns='vcard-temp:x:update'>
<photo>sha1-hash-of-image</photo>
</x>
</presence>
The following stanza contains two child elements, one of which is
qualified by the default namespace and one of which is qualified by
an extended namespace; the extension element in turn contains a child
elememnt that is qualified by a different extended namespace.
<message to='juliet@capulet.com'>
<body>Hello?</body>
<html xmlns='http://jabber.org/protocol/xhtml-im'>
<body xmlns='http://www.w3.org/1999/xhtml'>
<p style='font-weight:bold'>Hello?</t>
</body>
</html>
</message>
It is conventional in the XMPP community for implementations to not
generate namespace prefixes for elements that are qualified by
extended namespaces. However, if an implementation generates such
namespace prefixes then it MUST include the namespace declaration in
the stanza itself or a child element of the stanza, not in the stream
header (see Section 5.5.3).
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Routing entities (typically servers) SHOULD try to maintain prefixes
when serializing XML stanzas for processing, but receiving entities
MUST NOT rely on the prefix strings having any particular value.
Support for any given extended namespace is OPTIONAL on the part of
any implementation. If an entity does not understand such a
namespace, the entity's expected behavior depends on whether the
entity is (1) the recipient or (2) an entity that is routing the
stanza to the recipient.
Recipient: If a recipient receives a stanza that contains an element
or attribute it does not understand, it MUST silently ignore that
particular XML data, i.e., it MUST NOT process it or present it to
a user or associated application (if any). In particular:
* If an entity receives a message or presence stanza that
contains XML data qualified by a namespace it does not
understand, the portion of the stanza that qualified by the
unknown namespace MUST be ignored.
* If an entity receives a message stanza whose only child element
is qualified by a namespace it does not understand, it MUST
ignore the entire stanza.
* If an entity receives an IQ stanza of type "get" or "set"
containing a child element qualified by a namespace it does not
understand, the entity MUST return an IQ stanza of type "error"
with an error condition of <service-unavailable/>.
Router: If a routing entity (typically a server) handles a stanza
that contains a child element it does not understand, it MUST
ignore the associated XML data by routing or delivering it
untouched to the recipient.
9.5. Stanza Size
XMPP is optimized for the exchange of relatively large numbers of
relatively small stanzas. A client or server MAY enforce a maximum
stanza size. The maximum stanza size MUST NOT be smaller than 10000
bytes, from the opening "<" character to the closing ">" character.
If an entity receives a stanza that exceeds its maximum stanza size,
it MUST return a <not-acceptable/> stanza error or a <policy-
violation/> stream error.
10. Examples10.1. Client-to-Server
The following examples show the XMPP data flow for a client
negotiating an XML stream with a server, exchanging XML stanzas, and
closing the negotiated stream. The server is "im.example.com", the
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server requires use of TLS, the client authenticates via the SASL
PLAIN mechanism as "juliet@im.example.com", and the client binds a
client-submitted resource to the stream. It is assumed that before
sending the initial stream header, the client has already resolved an
SRV record of _xmpp-client._tcp.im.example.com and has opened a TCP
connection to the advertised port at the resolved IP address.
Note: The alternate steps shown are provided only to illustrate
the protocol for failure cases; they are not exhaustive and would
not necessarily be triggered by the data sent in the examples.
10.1.1. TLS
Step 1: Client initiates stream to server:
C: <stream:stream
from='juliet@im.example.com'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
Step 2: Server responds by sending a response stream header to
client:
S: <stream:stream
from='im.example.com'
id='t7AMCin9zjMNwQKDnplntZPIDEI='
to='juliet@im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
Step 3: Server sends stream features to client (STARTTLS extension
only at this point):
S: <stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
</stream:features>
Step 4: Client sends STARTTLS command to server:
C: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
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Desiring to send no further messages, the client closes the stream.
C: </stream:stream>
Consistent with the recommended stream closing handshake, the server
closes the stream as well:
S: </stream:stream>
Client now terminates the underlying TCP connection.
10.2. Server-to-Server Examples
The following examples show the data flow for a server negotiating an
XML stream with another server, exchanging XML stanzas, and closing
the negotiated stream. The initiating server ("Server1") is
im.example.com; the receiving server ("Server2") is example.net and
it requires use of TLS; im.example.com presents a certificate and
authenticates via the SASL EXTERNAL mechanism. It is assumed that
before sending the initial stream header, Server1 has already
resolved an SRV record of _xmpp-server._tcp.example.net and has
opened a TCP connection to the advertised port at the resolved IP
address.
Note: The alternate steps shown are provided only to illustrate
the protocol for failure cases; they are not exhaustive and would
not necessarily be triggered by the data sent in the examples.
10.2.1. TLS
Step 1: Server1 initiates stream to Server2:
S1: <stream:stream
from='im.example.com'
to='example.net'
version='1.0'
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'>
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Desiring to send no further messages, Server1 closes the stream. (In
practice, the stream would most likely remain open for some time,
since Server1 and Server2 do not immediately know if the stream will
be needed for further communication.)
S1: </stream:stream>
Consistent with the recommended stream closing handshake, Server2
closes the stream as well:
S2: </stream:stream>
Server1 now terminates the underlying TCP connection.
11. Server Rules for Processing XML Stanzas
An XMPP server MUST ensure in-order processing of XML stanzas between
any two entities. This includes stanzas sent by a client to its
server for direct processing by the server (e.g., in-order processing
of a roster get and initial presence as described in [XMPP-IM]).
Beyond the requirement for in-order processing, each server
implementation will contain its own logic for processing stanzas it
receives. Such logic determines whether the server needs to ROUTE a
given stanza to another domain, DELIVER it to a local entity
(typically a connected client associated with a local account), or
HANDLE it directly within the server itself. The following rules
apply.
Note: Particular XMPP applications MAY specify delivery rules that
modify or supplement the following rules; for example, a set of
delivery rules for instant messaging and presence applications is
defined in [XMPP-IM].
11.1. No 'to' Address11.1.1. Overview
If the stanza possesses no 'to' attribute, the server MUST handle it
directly on behalf of the entity that sent it, where the meaning of
"handle it directly" depends on whether the stanza is message,
presence, or IQ. Because all stanzas received from other servers
MUST possess a 'to' attribute, this rule applies only to stanzas
received from a local entity (such as a client) that is connected to
the server.
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If the server receives a message stanza with no 'to' attribute, it
MUST treat the message as if the 'to' address were the bare JID
<localpart@domain> of the sending entity.
11.1.3. Presence
If the server receives a presence stanza with no 'to' attribute, it
MUST broadcast it to the entities that are subscribed to the sending
entity's presence, if applicable ([XMPP-IM] defines the semantics of
such broadcasting for presence applications).
11.1.4. IQ
If the server receives an IQ stanza with no 'to' attribute, it MUST
process the stanza on behalf of the account from which received the
stanza, as follows:
1. If the IQ stanza is of type "get" or "set" and the server
understands the namespace that qualifies the payload, the server
MUST handle the stanza on behalf of the sending entity or return
an appropriate error to the sending entity. While the meaning of
"handle" is determined by the semantics of the qualifying
namespace, in general the server will respond to the IQ stanza of
type "get" or "set" by returning an appropriate IQ stanza of type
"result" or "error", responding as if the server were the bare
JID of the sending entity. As an example, if the sending entity
sends an IQ stanza of type "get" where the payload is qualified
by the 'jabber:iq:roster' namespace (as described in [XMPP-IM]),
then the server will return the roster associated with the
sending entity's bare JID to the particular resource of the
sending entity that requested the roster.
2. If the IQ stanza is of type "get" or "set" and the server does
not understand the namespace that qualifies the payload, the
server MUST return an error to the sending entity, which MUST be
<service-unavailable/>.
3. If the IQ stanza is of type "error" or "result", the server MUST
handle the error or result as appropriate for the request-
response interaction, responding as if the server were the bare
JID of the sending entity.
11.2. Local Domain
If the hostname of the domainpart of the JID contained in the 'to'
attribute matches one of the configured hostnames of the server
itself, the server MUST first determine if the hostname is serviced
by the server or by a specialized local service. If the latter, the
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server MUST route the stanza to that service. If the former, the
server MUST proceed as follows.
11.2.1. Mere Domain
If the JID contained in the 'to' attribute is of the form <domain>,
then the server MUST either handle the stanza as appropriate for the
stanza kind or return an error stanza to the sender.
11.2.2. Domain with Resource
If the JID contained in the 'to' attribute is of the form <domain/
resource>, then the server MUST either handle the stanza as
appropriate for the stanza kind or return an error stanza to the
sender.
11.2.3. Localpart at Domain
Note: For addresses of this type, more detailed rules in the
context of instant messaging and presence applications are
provided in [XMPP-IM].
11.2.3.1. No Such User
If there is no local account associated with the <localpart@domain>,
how the stanza is processed depends on the stanza type.
o For a message stanza, the server MUST return a <service-
unavailable/> stanza error to the sender.
o For a presence stanza, the server SHOULD silently discard the
stanza.
o For an IQ stanza, the server MUST return a <service-unavailable/>
stanza error to the sender.
11.2.3.2. Bare JID
If the JID contained in the 'to' attribute is of the form
<localpart@domain>, how the stanza is processed depends on the stanza
type.
o For a message stanza, if there exists at least one connected
resource for the account the server SHOULD deliver it to at least
one of the connected resources. If there exists no connected
resource, the server MUST either return an error or store the
message offline for delivery when the account next has a connected
resource.
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o For a presence stanza, if there exists at least one connected
resource for the account the server SHOULD deliver it to at least
one of the connected resources. If there exists no connected
resource, the server MUST silently discard the stanza.
o For an IQ stanza, the server MUST handle it directly on behalf of
the intended recipient.
11.2.3.3. Full JID
If the JID contained in the 'to' attribute is of the form
<localpart@domain/resource> and there is no connected resource that
exactly matches the full JID, the stanza is processed as if the JID
were of the form <localpart@domain>.
If the JID contained in the 'to' attribute is of the form
<localpart@domain/resource> and there is a connected resource that
exactly matches the full JID, the server SHOULD deliver the stanza to
that connected resource.
11.3. Remote Domain
If the hostname of the domainpart of the JID contained in the 'to'
attribute does not match one of the configured hostnames of the
server itself, the server SHOULD attempt to route the stanza to the
remote domain (subject to local service provisioning and security
policies regarding inter-domain communication, since such
communication is optional for any given deployment). There are two
possible cases.
11.3.1. Existing Stream
If a server-to-server stream already exists between the two domains,
the sender's server will attempt to route the stanza to the
authoritative server for the remote domain over the existing stream.
11.3.2. No Existing Stream
If there exists no server-to-server stream between the two domains,
the sender's server will proceed as follows:
1. Resolve the hostname of the remote domain (as defined under
Section 14.4).
2. Negotiate a server-to-server stream between the two domains (as
defined under Section 6 and Section 7).
3. Route the stanza to the authoritative server for the remote
domain over the newly-established stream.
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If routing of a stanza to the intended recipient's server is
unsuccessful, the sender's server MUST return an error to the sender.
If resolution of the remote domain is unsuccessful, the stanza error
MUST be <remote-server-not-found/>. If resolution succeeds but
streams cannot be negotiated, the stanza error MUST be <remote-
server-timeout/>.
If stream negotiation with the intended recipient's server is
successful but the remote server cannot deliver the stanza to the
recipient, the remote server MUST return an appropriate error to the
sender by way of the sender's server.
12. XML Usage12.1. Restrictions
The Extensible Messaging and Presence Protocol (XMPP) defines a class
of data objects called XML streams as well as the behavior of
computer programs that process XML streams. XMPP is an application
profile or restricted form of the Extensible Markup Language [XML],
and a complete XML stream (including start and end stream tags) is a
conforming XML document.
However, XMPP does not deal with XML documents but with XML streams.
Because XMPP does not require the parsing of arbitrary and complete
XML documents, there is no requirement that XMPP needs to support the
full feature set of [XML]. In particular, the following features of
XML are prohibited in XMPP:
o comments (as defined in Section 2.5 of [XML])
o processing instructions (Section 2.6 therein)
o internal or external DTD subsets (Section 2.8 therein)
o internal or external entity references (Section 4.2 therein) with
the exception of the predefined entities (Section 4.6 therein)
An XMPP implementation MUST behave as follows with regard to these
features:
1. An XMPP implementation MUST NOT inject characters matching such
features into an XML stream.
2. If an XMPP implementation receives characters matching such
features over an XML stream, it MUST return a stream error, which
SHOULD be <restricted-xml/> but MAY be <bad-format/>.
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XML namespaces (see [XML-NAMES]) are used within XMPP streams to
create strict boundaries of data ownership. The basic function of
namespaces is to separate different vocabularies of XML elements that
are structurally mixed together. Ensuring that XMPP streams are
namespace-aware enables any allowable XML to be structurally mixed
with any data element within XMPP. XMPP-specific rules for XML
namespace names and prefixes are defined under Section 5.5 for XML
streams and Section 9.4 for XML stanzas.
12.3. Well-Formedness
There are two varieties of well-formedness:
o "XML-well-formedness" in accordance with the definition of "well-
formed" in Section 2.1 of [XML].
o "Namespace-well-formedness" in accordance with the definition of
"namespace-well-formed" in Section 7 of [XML-NAMES].
The following rules apply.
An XMPP entity MUST NOT generate data that is not XML-well-formed.
An XMPP entity MUST NOT accept data that is not XML-well-formed;
instead it MUST return an <xml-not-well-formed/> stream error and
close the stream over which the data was received.
An XMPP entity MUST NOT generate data that is not namespace-well-
formed. An XMPP server SHOULD NOT route or deliver data that is not
namespace-well-formed, and SHOULD return a stanza error of <not-
acceptable/> or a stream error of <xml-not-well-formed/> in response
to the receipt of such data.
Note: Because these restrictions were underspecified in the
predecessor to this specification, it is possible that
implementations based on that predecessor will send data that does
not comply with the restrictions.
12.4. Validation
A server is not responsible for ensuring that XML data delivered to a
client or routed to another server is valid, in accordance with the
definition of "valid" provided in Section 2.8 of [XML]. An
implementation MAY choose to accept or provide only validated data,
but such behavior is OPTIONAL. A client SHOULD NOT rely on the
ability to send data that does not conform to the schemas, and SHOULD
ignore any non-conformant elements or attributes on the incoming XML
stream.
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Note: The terms "valid" and "well-formed" are distinct in XML.
12.5. Inclusion of XML Declaration
Before sending a stream header, an implementation SHOULD send an XML
declaration (matching production [23] content of [XML]).
Applications MUST follow the rules provided in [XML] regarding the
format of the XML declaration and the circumstances under which the
XML declaration is included.
12.6. Character Encoding
Implementations MUST support the UTF-8 transformation of Universal
Character Set [UCS2] characters, as needed for conformance with
[CHARSET] and as defined in [UTF-8]. Implementations MUST NOT
attempt to use any other encoding. If one party to an XML stream
detects that the other party has attempted to send XML data with an
encoding other than UTF-8, it MUST return a stream error, which
SHOULD be <unsupported-encoding/> but MAY be <bad-format/>.
Note: Because it is mandatory for an XMPP implementation to support
all and only the UTF-8 encoding and because UTF-8 always has the same
byte order, an implementation MUST NOT send a byte order mark ("BOM")
at the beginning of the data stream. If an entity receives the
Unicode character U+FEFF anywhere in an XML stream (including as the
first character of the stream), it MUST interpret that character as a
zero width no-break space, not as a byte order mark.
12.7. Whitespace
Except where explicitly disallowed (e.g., during TLS negotiation
(Section 6) and SASL negotiation (Section 7)), either entity MAY send
whitespace as separators between XML stanzas or between any other
first-level elements sent over the stream. One common use for
sending such whitespace is explained under Section 5.3.3.
12.8. XML Versions
XMPP is an application profile of XML 1.0. A future version of XMPP
might be defined in terms of higher versions of XML, but this
specification addresses XML 1.0 only.
13. Internationalization Considerations
As specified under Section 12.6, XML streams MUST be encoded in
UTF-8.
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As specified under Section 5.4, an XML stream SHOULD include an 'xml:
lang' attribute specifying the default language for any XML character
data that is intended to be presented to a human user. As specified
under Section 9.1.5, an XML stanza SHOULD include an 'xml:lang'
attribute if the stanza contains XML character data that is intended
to be presented to a human user. A server SHOULD apply the default
'xml:lang' attribute to stanzas it routes or delivers on behalf of
connected entities, and MUST NOT modify or delete 'xml:lang'
attributes on stanzas it receives from other entities.
As specified under Section 3, a server MUST support and enforce
[IDNA] for domainparts, the Nodeprep (Appendix A) profile of
[STRINGPREP] for localparts, and the Resourceprep (Appendix B)
profile of [STRINGPREP] for resourceparts; this enables XMPP
addresses to include a wide variety of Unicode characters outside the
US-ASCII range.
14. Security Considerations14.1. High Security
For the purposes of XMPP communication (client-to-server and server-
to-server), the term "high security" refers to the use of security
technologies that provide both mutual authentication and integrity
checking (on the importance of high security, refer to [STRONGSEC]);
in particular, when using certificate-based authentication to provide
high security, a trust chain SHOULD be established out-of-band,
although a shared certification authority signing certificates could
allow a previously unknown certificate to establish trust in-band.
See Section 14.2 regarding certificate validation procedures.
Implementations MUST support high security. Service provisioning
SHOULD use high security, subject to local security policies.
The initial stream and the response stream MUST be secured
separately, although security in both directions MAY be established
via mechanisms that provide mutual authentication.
14.2. Certificates
Channel encryption of an XML stream using Transport Layer Security as
described under Section 6, and in some cases also authentication as
described under Section 7, is commonly based on a digital certificate
presented by the receiving entity (or, in the case of mutual
authentication, both the receiving entity and the initiating entity).
This section describes best practices regarding the generation of
digital certificates to be presented by XMPP entities and the
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verification of digital certificates presented by XMPP entities.
Considerations specific to certificate geneneration and validation
with regard to client certificates or server certificates are
described in the following sections.
14.2.1. Certificate Generation14.2.1.1. General Considerations
The following rules apply to public key certificates that are issued
to XMPP entities:
1. The certificate MUST conform to [X509].
2. The certificate MUST NOT contain a basicConstraints extension
with the cA boolean set to TRUE.
3. The subject field MUST NOT be null.
4. The hash algorithm for the signature SHOULD be SHA-256 as defined
by [X509-ALGO].
5. The certificate SHOULD include an Authority Information Access
(AIA) extension that specifies the address of an Online
Certificate Status Protocol [OCSP] responder.
The following rules apply to issuers of XMPP certificates:
1. The certificate MUST conform to [X509].
2. The certificate MUST contain a keyUsage extension with the
digitalSignature bit set.
3. The subject field MUST NOT be null.
4. The hash algorithm for the signature SHOULD be SHA-256 as defined
by [X509-ALGO].
5. For issuers of public key certificates, the issuer's certificate
MUST contain a basicConstraints extension with the cA boolean set
to TRUE.
6. For issuers of access certificates, the issuer's certificate MUST
NOT contain a basicConstraints extension with the cA boolean set
to TRUE.
14.2.1.2. Server Certificates
In a digital certificate to be presented by an XMPP server (i.e., a
SERVER CERTIFICATE), it is RECOMMENDED for the certificate to include
one or more JIDs (i.e., domainparts) associated with domains serviced
at the server. The representations described in the following
sections are RECOMMENDED. These representations are provided in
preference order.
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A server's domainpart SHOULD be represented as an SRVName, i.e., as
an otherName field of type "id-on-dnsSRV" as specified in [X509-SRV].
14.2.1.2.2. dNSName
A server's domainpart SHOULD be represented as a dNSName, i.e., as a
subjectAltName extension of type dNSName.
The dNSName MAY contain one instance of the wildcard character '*'.
The wildcard character applies only to the left-most domain name
component and matches any single component (thus a dNSName of
*.example.com matches foo.example.com but not bar.foo.example.com or
example.com itself). The wildcard character is not allowed in
component fragments (thus a dNSName of im*.example.net is not allowed
and SHALL NOT be taken to match im1.example.net and im2.example.net).
14.2.1.2.3. XmppAddr
A server's domainpart MAY be represented as an XmppAddr, i.e., as a
UTF8String within an otherName entity inside the subjectAltName,
using the [ASN.1] Object Identifier "id-on-xmppAddr" specified under
Section 14.2.1.4. In server certificates, this representation is
included only for the sake of backward-compatibility.
14.2.1.2.4. Common Name
A server's domainpart SHOULD NOT be represented as a Common Name;
instead, the Common Name field SHOULD be reserved for representation
of a human-friendly name.
14.2.1.2.5. Examples
For our first (relatively simple) example, consider a company called
"Example Products, Inc." It hosts an XMPP service at
"im.example.com" (i.e., user addresses at the service are of the form
"user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-
server services at "im.example.com" yield one machine, called
"x.example.com", as follows:
_xmpp-client._tcp.im.example.com. 400 IN SRV 20 0 5222 x.example.com
_xmpp-server._tcp.im.example.com. 400 IN SRV 20 0 5269 x.example.com
The certificate presented by x.example.com contains the following
representations:
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o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-client.im.example.com"
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-server.im.example.com"
o A dNSName containing an ASCII string of "im.example.com"
o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of: "im.example.com"
o A CN containing an ASCII string of "Example Products, Inc."
For our second (more complex) example, consider an ISP called
"Example Internet Services". It hosts an XMPP service at
"example.net" (i.e., user addresses at the service are of the form
"user@example.net"), but SRV lookups for the xmpp-client and xmpp-
server services at "example.net" yield two machines ("x1.example.net"
and "x2.example.net"), as follows:
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
_xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
_xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.
Example Internet Services also hosts chatrooms at chat.example.net,
and provides an xmpp-server SRV record for that service as well (thus
enabling entity from remote domains to access that service). It also
might provide other such services in the future, so it wishes to
represent a wildcard in its certificate to handle such growth.
The certificate presented by either x1.example.net or x2.example.net
contains the following representations:
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-client.example.net"
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-server.example.net"
o An otherName type of SRVName (id-on-dnsSRV) containing an
IA5String (ASCII) string of: "_xmpp-server.chat.example.net"
o A dNSName containing an ASCII string of "example.net"
o A dNSName containing an ASCII string of "*.example.net"
o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of: "example.net"
o An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
string of: "chat.example.net"
o A CN containing an ASCII string of "Example Internet Services"
14.2.1.3. Client Certificates
In a digital certificate to be presented by an XMPP client controlled
by a human user (i.e., a CLIENT CERTIFICATE), it is RECOMMENDED for
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the certificate to include one or more JIDs associated with an XMPP
user. If included, a JID MUST be represented as an XmppAddr, i.e.,
as a UTF8String within an otherName entity inside the subjectAltName,
using the [ASN.1] Object Identifier "id-on-xmppAddr" specified under
Section 14.2.1.4.
14.2.1.4. ASN.1 Object Identifier
The [ASN.1] Object Identifier "id-on-xmppAddr" (also called an
XmppAddr) is defined as follows.
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 } -- other name forms
id-on-xmppAddr OBJECT IDENTIFIER ::= { id-on 5 }
XmppAddr ::= UTF8String
As an alternative to the "id-on-xmppAddr" notation, this Object
Identifier MAY be represented in dotted display format (i.e.,
"1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation
specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").
Thus for example the JID "juliet@im.example.com" as included in a
certificate could be formatted in any of the following three ways:
id-on-xmppAddr:
subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@im.example.com
dotted display format: subjectAltName=otherName:
1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com
URN notation: subjectAltName=otherName:urn:oid:
1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com
Use of the "id-on-xmppAddr" format is RECOMMENDED in the generation
of certificates, but all three formats MUST be supported for the
purpose of certificate validation.
The "id-on-xmppAddr" object identifier MAY be used on conjuction with
the extended key usage extension specified in Section 4.2.1.12 of
[X509] in order to explicitly define and limit the intended use of a
certificate to the XMPP network.
14.2.2. Certificate Validation
When an XMPP entity is presented with a server certificate or client
certificate by a peer for the purpose of encryption or authentication
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of XML streams as described under Section 6 and Section 7, the entity
MUST validate the certificate to determine if the certificate will be
considered a TRUSTED CERTIFICATE, i.e., a certificate that is
acceptable for encryption and/or authentication in accordance with
the XMPP entity's local service policies or configured settings.
For both server certificates and client certificates, the validating
entity MUST verify the integrity of the certificate, MUST verify that
the certificate has been properly signed by the issuing Certificate
Authority, and MUST support certificate revocation messages. An
implementation MUST enable a human user to view information about the
full chain of certificates.
The following sections describe certificate validation rules for
server-to-server and client-to-server streams.
14.2.2.1. Server Certificates
When an entity (client or server) validates a certificate presented
by an XMPP server, there are three possible cases, as discussed in
the following sections.
14.2.2.1.1. Case #1
If the server certificate appears to be certified by a chain of
certificates terminating in a trust anchor (as described in Section6.1 of [X509]), the entity MUST check the certificate for any
instances of the SRVName, dNSName, and XmppAddr (in that order of
preference) as described under Section 14.2.1.2.1,
Section 14.2.1.2.2, and Section 14.2.1.2.3. There are three possible
sub-cases:
Sub-Case #1: The server's certificate includes at least one
presented identity that matches the reference identity to which
the entity attempted to connect, in accordance with the matching
rules specified in [TLS-CERTS]. In this case, the entity MUST use
the matched domainpart as the validated identity of the XMPP
server.
Sub-Case #2: The server's certificate includes no presented identity
that matches the reference identity to which the entity attempted
to connect and a human user has not permanently accepted the
certificate during a previous connection attempt. In this case,
the entity MUST NOT use the presented domainpart (if any) as the
validated identity of the XMPP server. Instead, if the connecting
entity is a user-oriented client then it MUST either (1)
automatically terminate the connection with a bad certificate
error or (2) show the certificate (including the entire
certificate chain) to the user and give the user the choice of
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terminating the connecting or accepting the certificate
temporarily (i.e., for this connection attempt only) or
permanently (i.e., for all future connection attempts) and then
continuing with the connection; if a user permanently accepts a
certificate in this way, the client MUST cache the certificate (or
some non-forgeable representation such as a hash value) and in
future connection attempts behave as in Sub-Case #3. (It is the
resposibility of the human user to verify the hash value or
fingerprint of the certificate with the peer over a trusted
communication layer.) If the connecting entity is an XMPP server
or an automated client, the application SHOULD terminate the
connection (with a bad certificate error) and log the error to an
appropriate audit log; an XMPP server or automated client MAY
provide a configuration setting that disables this check, but MUST
enable the check by default.
Sub-Case #3: The server's certificate includes no presented identity
that matches the reference identity to which the entity attempted
to connect but a human user has permanently accepted the
certificate during a previous connection attempt; the entity MUST
verify that the cached certificate was presented and MUST notify
the user if the certificate has changed.
14.2.2.1.2. Case #2
If the server certificate is certified by a Certificate Authority not
known to the entity, the entity MUST proceed as under Case #1, Sub-
Case #2 or Case #1, Sub-Case #3 as appropriate.
14.2.2.1.3. Case #3
If the server certificate is self-signed, the entity MUST proceed as
under Case #1, Sub-Case #2 or Case #1, Sub-Case #3 as appropriate.
14.2.2.2. Client Certificates
When an XMPP server validates a certificate presented by a client,
there are three possible cases, as discussed in the following
sections.
14.2.2.2.1. Case #1
If the client certificate appears to be certified by a chain of
certificates terminating in a trust anchor (as described in Section6.1 of [X509]), the server MUST check the certificate for any
instances of the XmppAddr as described under Section 14.2.1.4. There
are three possible sub-cases:
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Sub-Case #1: The server finds one XmppAddr for which the domainpart
of the represented JID matches one of the configured hostnames of
the server itself; the server SHOULD use this represented JID as
the validated identity of the client.
Sub-Case #2: The server finds more than one XmppAddr for which the
domainpart of the represented JID matches one of the configured
hostnames of the server itself; the server SHOULD use one of these
represented JIDs as the validated identity of the client, choosing
among them according to local service policies or based on the
'to' address of the initial stream header.
Sub-Case #3: The server finds no XmppAddrs, or finds at least one
XmppAddr but the domainpart of the represented JID does not match
one of the configured hostnames of the server itself; the server
MUST NOT use the represented JID (if any) as the validated
identity of the client but instead MUST either validate the
identity of the client using other means.
14.2.2.2.2. Case #2
If the client certificate is certified by a Certificate Authority not
known to the server, the server MUST proceed as under Case #1, Sub-
Case #3.
14.2.2.2.3. Case #3
If the client certificate is self-signed, the server MUST proceed as
under Case #1, Sub-Case #3.
14.2.2.3. Checking of Certificates in Long-Lived Streams
Because XMPP uses long-lived XML streams, it is possible that a
certificate presented during stream negotiation might expire or be
revoked while the stream is still live (this is especially relevant
in the context of server-to-server streams). Therefore, each party
to a long-lived stream SHOULD:
1. Cache the expiration date of the certificate presented by the
other party and any certificates on which that certificate
depends (such as a root or intermediate certificate for a
certification authority), and terminate the stream when any such
certificate expires.
2. Periodically query the Online Certificate Status Protocol [OCSP]
responder listed in the Authority Information Access (AIA)
extension of the certificate presented by the other party and any
certificates on which that certificate depends (such as a root or
intermediate certificate for a certification authority), and
terminate the stream if any such certificate has been revoked.
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If the identity presented by the initiating entity changes materially
between the old stream and the new stream, then the receiving entity
MUST perform a full re-authentication on the new stream.
14.2.2.4. Use of Certificates in XMPP Extensions
Certificates MAY be used in extensions to XMPP for the purpose of
application-layer encryption or authentication above the level of XML
streams (e.g., for end-to-end encryption). Such extensions will
define their own certificate handling rules, which at a minimum
SHOULD be consistent with the rules specified herein but MAY specify
additional rules.
14.3. Client-to-Server Communication
A compliant client implementation MUST support both TLS and SASL for
connections to a server.
The TLS protocol for encrypting XML streams (defined under Section 6)
provides a reliable mechanism for helping to ensure the
confidentiality and integrity of data exchanged between two entities.
The SASL protocol for authenticating XML streams (defined under
Section 7) provides a reliable mechanism for validating that a client
connecting to a server is who it claims to be.
Client-to-server communication MUST NOT proceed until the DNS
hostname asserted by the server has been resolved as specified under
Section 4. If there is a mismatch between the hostname to which a
client attempted to connect (e.g., "example.net") and the hostname to
which the client actually connects (e.g., "x1.example.net"), the
client MUST warn a human user about the mismatch and the human user
MUST approve the connection before the client proceeds; however, the
client MAY also allow the user to add the presented hostname to a
configured set of accepted hostnames to expedite future connections.
A client's IP address and method of access MUST NOT be made public by
a server, nor are any connections other than the original server
connection necessary. This helps to protect the client's server from
direct attack or identification by third parties.
14.4. Server-to-Server Communication
A compliant server implementation MUST support both TLS and SASL for
inter-domain communication.
Because service provisioning is a matter of policy, it is optional
for any given domain to communicate with other domains, and server-
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to-server communication can be disabled by the administrator of any
given deployment. If a particular domain enables inter-domain
communication, it SHOULD enable high security.
Administrators might want to require use of SASL for server-to-server
communication to ensure both authentication and confidentiality
(e.g., on an organization's private network). Compliant
implementations SHOULD support SASL for this purpose.
Server-to-server communication MUST NOT proceed until the DNS
hostnames asserted by both servers have been resolved as specified
under Section 4.
14.5. Order of Layers
The order of layers in which protocols MUST be stacked is:
1. TCP
2. TLS
3. SASL
4. XMPP
The rationale for this order is that [TCP] is the base connection
layer used by all of the protocols stacked on top of TCP, [TLS] is
often provided at the operating system layer, [SASL] is often
provided at the application layer, and XMPP is the application
itself.
14.6. Mandatory-to-Implement Technologies
At a minimum, all implementations MUST support the following
mechanisms unless otherwise specified below:
for authentication only: the SASL Salted Challenge Response
mechanism [SCRAM] (preferred) and the SASL PLAIN mechanism [PLAIN]
(not preferred
for confidentiality only: TLS (using the
TLS_RSA_WITH_AES_128_CBC_SHA cipher)
for both confidentiality and authentication with passwords: TLS plus
the SASL Salted Challenge Response mechanism (see [SCRAM]) and TLS
plus the SASL PLAIN mechanism (see [PLAIN])
for both confidentiality and authentication without passwords: TLS
plus the SASL EXTERNAL mechanism (see Appendix A of [SASL]) using
the TLS_RSA_WITH_AES_128_CBC_SHA cipher supporting peer
certificates (clients SHOULD support this, and servers MUST)
Naturally, implementations MAY support other ciphers with TLS and MAY
support other SASL mechanisms.
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Note: The use of TLS plus SASL SCRAM or SASL PLAIN replaces the
SASL DIGEST-MD5 mechanism as XMPP's mandatory-to-implement
password-based method for authentication. For backward-
compatibility with existing deployed infrastructure,
implementations are encouraged to continue supporting the SASL
DIGEST-MD5 mechanism as specified in [DIGEST-MD5], however there
are known interoperability issues with DIGEST-MD5 that make it
impractical in the long term. The use of the SASL SCRAM mechanism
is strongly preferred over the SASL PLAIN mechanism because of its
superior security properties, and PLAIN is intended to be a
fallback only for implementations that do not yet support SCRAM.
Refer to [SCRAM] and [PLAIN] for important security considerations
related to these SASL mechanisms.
14.7. Hash Function Agility
XMPP itself does not directly mandate the use of any particular hash
function. However, technologies on which XMPP depends (e.g., TLS and
particular SASL mechanisms), as well as various XMPP extensions,
might make use of hash functions. Those who implement XMPP
technologies or who develop XMPP extensions are advised to closely
monitor the state of the art regarding attacks against cryptographic
hashes in Internet protocols as they relate to XMPP. For helpful
guidance, refer to [HASHES].
14.8. SASL Downgrade Attacks
Because the initiating entity chooses an acceptable SASL mechanism
from the list presented by the receiving entity, the initiating
entity depends on the receiving entity's list for authentication.
This dependency introduces the possibility of a downgrade attack if
an attacker can gain control of the channel and therefore present a
weak list of mechanisms. To prevent this attack, the parties SHOULD
protect the channel using TLS before attempting SASL negotiation.
14.9. Lack of SASL Channel Binding to TLS
The SASL framework itself does not provide a method for binding SASL
authentication to a security layer providing confidentiality and
integrity protection that was negotiated at a lower layer. Such a
binding is known as a "channel binding" (see [CHANNEL]). Some SASL
mechanisms provide channel bindings. However, if a SASL mechanism
does not provide a channel binding, then the mechanism cannot provide
a way to verify that the source and destination end points to which
the lower layer's security is bound are equivalent to the end points
that SASL is authenticating; furthermore, if the end points are not
identical, then the lower layer's security cannot be trusted to
protect data transmitted between the SASL-authenticated entities. In
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such a situation, a SASL security layer SHOULD be negotiated that
effectively ignores the presence of the lower-layer security.
14.10. Use of base64 in SASL
Both the client and the server MUST verify any base64 data received
during SASL negotiation (Section 7). An implementation MUST reject
(not ignore) any characters that are not explicitly allowed by the
base64 alphabet; this helps to guard against creation of a covert
channel that could be used to "leak" information.
An implementation MUST NOT break on invalid input and MUST reject any
sequence of base64 characters containing the pad ('=') character if
that character is included as something other than the last character
of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
buffer overflow attacks and other attacks on the implementation.
While base 64 encoding visually hides otherwise easily recognized
information (such as passwords), it does not provide any
computational confidentiality.
All uses of base 64 encoding MUST follow the definition in Section 4
of [BASE64] and padding bits MUST be set to zero.
14.11. Stringprep Profiles
XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for
processing of domainparts; for security considerations related to
Nameprep, refer to the appropriate section of [NAMEPREP].
In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
(Appendix A) for localparts and Resourceprep (Appendix B) for
resourceparts.
The Unicode and ISO/IEC 10646 repertoires have many characters that
look similar. In many cases, users of security protocols might
perform visual matching, such as when comparing the names of trusted
third parties. Because it is impossible to map similar-looking
characters without a great deal of context (such as knowing the fonts
used), stringprep does nothing to map similar-looking characters
together, nor to prohibit some characters because they look like
others.
A localpart can be employed as one part of an entity's address in
XMPP. One common usage is as the username of an instant messaging
user; another is as the name of a multi-user conference room; and
many other kinds of entities could use localparts as part of their
addresses. The security of such services could be compromised based
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on different interpretations of the internationalized localpart; for
example, a user entering a single internationalized localpart could
access another user's account information, or a user could gain
access to a hidden or otherwise restricted chat room or service.
A resourcepart can be employed as one part of an entity's address in
XMPP. One common usage is as the name for an instant messaging
user's connected resource; another is as the nickname of a user in a
multi-user conference room; and many other kinds of entities could
use resourceparts as part of their addresses. The security of such
services could be compromised based on different interpretations of
the internationalized resourcepart; for example, a user could attempt
to initiate multiple connections with the same name, or a user could
send a message to someone other than the intended recipient in a
multi-user conference room.
14.12. Address Spoofing
As discussed in [XEP-0165], there are two forms of address spoofing:
forging and mimicking.
14.12.1. Address Forging
In the context of XMPP technologies, address forging occurs when an
entity is able to generate an XML stanza whose 'from' address does
not correspond to the account credentials with which the entity
authenticated onto the network (or an authorization identity provided
during SASL negotiation (Section 7)). For example, address forging
occurs if an entity that authenticated as "juliet@im.example.com" is
able to send XML stanzas from "nurse@im.example.com" or
"romeo@example.net".
Address forging is difficult in XMPP systems, given the requirement
for sending servers to stamp 'from' addresses and for receiving
servers to verify sending domains via server-to-server
authentication. However, address forging is not impossible, since a
rogue server could forge JIDs at the sending domain by ignoring the
stamping requirement. A rogue server could even forge JIDs at other
domains by means of a DNS poisoning attack if [DNSSEC] is not used.
This specification does not define methods for discovering or
counteracting such rogue servers.
Note: An entity outside the security perimeter of a particular server
cannot reliably distinguish between bare JIDs of the form
<localpart@domain> at that server, since the server could forge any
such JID; therefore only the domainpart can be authenticated or
authorized with any level of assurance.
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Address mimicking occus when an entity provides legitimate
authentication credentials for and sends XML stanzas from an account
whose JID appears to a human user to be the same as another JID. For
example, in some XMPP clients the address "paypa1@example.org"
(spelled with the number one as the final character of the localpart)
might appear to be the same as "paypal@example.org (spelled with the
lower-case version of the letter "L"), especially on casual visual
inspection; this phenomenon is sometimes called "typejacking". A
more sophisticated example of address mimicking might involve the use
of characters from outside the US-ASCII range, such as the Cherokee
characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 instead
of the US-ASCII characters "STPETER".
In some examples of address mimicking, it is unlikely that the
average user could tell the difference between the real JID and the
fake JID. (Naturally, there is no way to distinguish with full
certainty which is the fake JID and which is the real JID; in some
communication contexts, the JID with Cherokee characters might be the
real JID and the JID with US-ASCII characters might thus appear to be
the fake JID.) Because JIDs can contain almost any Unicode
character, it can be relatively easy to mimic some JIDs in XMPP
systems. The possibility of address mimicking introduces security
vulnerabilities of the kind that have also plagued the World Wide
Web, specifically the phenomenon known as phishing.
Mimicked addresses that involve characters from only one character
set or from the character set typically employed by a particular user
are not easy to combat (e.g., the simple typejacking attack
previously described, which relies on a surface similarity between
the characters "1" and "l" in some presentations). However, mimicked
addresses that involve characters from more than one character set,
or from a character set not typically employed by a particular user,
can be mitigated somewhat through intelligent presentation. In
particular, every human user of an XMPP technology presumably has a
preferred language (or, in some cases, a small set of preferred
languages), which an XMPP application SHOULD gather either explicitly
from the user or implicitly via the operating system of the user's
device. Furthermore, every language has a range (or a small set of
ranges) of characters normally used to represent that language in
textual form. Therefore, an XMPP application SHOULD warn the user
when presenting a JID that uses characters outside the normal range
of the user's preferred language(s). This recommendation is not
intended to discourage communication across language communities;
instead, it recognizes the existence of such language communities and
encourages due caution when presenting unfamiliar character sets to
human users.
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For more detailed recommendations regarding prevention of address
mimicking in XMPP systems, refer to [XEP-0165].
14.13. Firewalls
Communication using XMPP normally occurs over TCP connections on port
5222 (client-to-server) or port 5269 (server-to-server), as
registered with the IANA (see Section 15). Use of these well-known
ports allows administrators to easily enable or disable XMPP activity
through existing and commonly-deployed firewalls.
14.14. Denial of Service
[DOS] defines denial of service as follows:
A Denial-of-Service (DoS) attack is an attack in which one or more
machines target a victim and attempt to prevent the victim from
doing useful work. The victim can be a network server, client or
router, a network link or an entire network, an individual
Internet user or a company doing business using the Internet, an
Internet Service Provider (ISP), country, or any combination of or
variant on these.
[XEP-0205] provides a detailed discussion of potential denial of
service attacks against XMPP systems and best practices for
preventing such attacks. The recommendations include:
1. A server implementation SHOULD enable a server administrator to
limit the number of TCP connections that it will accept from a
given IP address at any one time. If an entity attempts to
connect but the maximum number of TCP connections has been
reached, the receiving server MUST NOT allow the new connection
to proceed.
2. A server implementation SHOULD enable a server administrator to
limit the number of TCP connection attempts that it will accept
from a given IP address in a given time period. (While it is
possible to limit the number of connections at the TCP layer
rather than at the XMPP application layer, this is not advisable
because limits at the TCP layer might result in an inability to
access non-XMPP services.) If an entity attempts to connect but
the maximum number of connections has been reached, the receiving
server MUST NOT allow the new connection to proceed.
3. A server MUST NOT process XML stanzas from clients that have not
yet provided appropriate authentication credentials and MUST NOT
process XML stanzas from peer servers whose identity it has not
either authenticated via SASL or weakly verified via server
dialback (see [XEP-0220]).
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4. A server implementation SHOULD enable a server administrator to
limit the number of connected resources it will allow an account
to bind at any one time. If a client attempts to bind a resource
but it has already reached the configured number of allowable
resources, the receiving server MUST return a <resource-
constraint/> stanza error.
5. A server implementation SHOULD enable a server administrator to
limit the size of stanzas it will accept from a connected client
or peer server. If a connected resource or peer server sends a
stanza that violates the upper limit, the receiving server SHOULD
NOT process the stanza and instead SHOULD return a <not-allowed/>
stanza error. Alternatively (e.g., if the sender has sent an
egregiously large stanza), the server MAY instead return a
<policy-violation/> stream error.
6. A server implementation SHOULD enable a server administrator to
limit the number of XML stanzas that a connected client is
allowed to send to distinct recipients within a given time
period. If a connected client sends too many stanzas to distinct
recipients in a given time period, the receiving server SHOULD
NOT process the stanza and instead SHOULD return an <unexpected-
request/> stanza error.
7. A server implementation SHOULD enable a server administrator to
limit the amount of bandwidth it will allow a connected client or
peer server to use in a given time period.
8. A server implementation MAY enable a server administrator to
limit the types of stanzas (based on the extended content
"payload") that it will allow a connected resource or peer server
send over an active connection. Such limits and restrictions are
a matter of deployment policy.
9. A server implementation MAY refuse to route or deliver any stanza
that it considers to be abusive, with or without returning an
error to the sender.
For more detailed recommendations regarding denial of service attacks
in XMPP systems, refer to [XEP-0205].
14.15. Presence Leaks
One of the core aspects of XMPP is presence: information about the
network availability of an XMPP entity (i.e., whether the entity is
currently online or offline). A PRESENCE LEAK occurs when an
entity's network availability is inadvertently and involuntarily
revealed to a second entity that is not authorized to know the first
entity's network availability.
Although presence is discussed more fully in [XMPP-IM], it is
important to note that an XMPP server MUST NOT leak presence. In
particular at the core XMPP level, real-time addressing and network
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availability is associated with a specific connected resource;
therefore, any disclosure of a connected resource's full JID
comprises a presence leak. To help prevent such a presence leak, a
server MUST NOT return different stanza errors if a potential
attacker sends XML stanzas to the entity's bare JID
(<localpart@domain>) or full JID (<localpart@domain/resource>).
14.16. Directory Harvesting
When a server generates an error stanza in response to receiving a
stanza for a user account that does not exist, the use of the
<service-unavailable/> stanza error condition can help protect
against dictionary attacks, since this is the same error condition
that is returned if, for instance, the namespace of an IQ child
element is not understood, or if offline message storage or message
forwarding is not enabled for a domain. However, subtle differences
in the exact XML of error stanzas, as well as in the timing with
which such errors are returned, can enable an attacker to determine
the network presence of a user when more advanced blocking
technologies are not used (see for instance [XEP-0016] and
[XEP-0191]).
15. IANA Considerations
The following sections update the registrations provided in
[RFC3920].
15.1. XML Namespace Name for TLS Data
A URN sub-namespace for STARTTLS negotiation data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-tls
Specification: XXXX
Description: This is the XML namespace name for STARTTLS negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
15.2. XML Namespace Name for SASL Data
A URN sub-namespace for SASL negotiation data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
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URI: urn:ietf:params:xml:ns:xmpp-sasl
Specification: XXXX
Description: This is the XML namespace name for SASL negotiation
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
15.3. XML Namespace Name for Stream Errors
A URN sub-namespace for stream error data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-streams
Specification: XXXX
Description: This is the XML namespace name for stream error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
15.4. XML Namespace Name for Resource Binding
A URN sub-namespace for resource binding in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-bind
Specification: XXXX
Description: This is the XML namespace name for resource binding in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
15.5. XML Namespace Name for Stanza Errors
A URN sub-namespace for stanza error data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-stanzas
Specification: XXXX
Description: This is the XML namespace name for stanza error data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
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Registrant Contact: IETF, XMPP Working Group, <xmppwg@xmpp.org>
15.6. Nodeprep Profile of Stringprep
The Nodeprep profile of stringprep is defined under Nodeprep
(Appendix A). The IANA has registered Nodeprep in the stringprep
profile registry.
Name of this profile:
Nodeprep
RFC in which the profile is defined:
XXXX
Indicator whether or not this is the newest version of the profile:
This is the first version of Nodeprep
15.7. Resourceprep Profile of Stringprep
The Resourceprep profile of stringprep is defined under Resourceprep
(Appendix B). The IANA has registered Resourceprep in the stringprep
profile registry.
Name of this profile:
Resourceprep
RFC in which the profile is defined:
XXXX
Indicator whether or not this is the newest version of the profile:
This is the first version of Resourceprep
15.8. GSSAPI Service Name
The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as
defined under Section 7.5.
15.9. Port Numbers
The IANA has registered "xmpp-client" and "xmpp-server" as keywords
for [TCP] ports 5222 and 5269 respectively.
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These ports SHOULD be used for client-to-server and server-to-server
communications respectively, but other ports MAY be used.
16. Conformance Requirements
This section describes a protocol feature set that summarizes the
conformance requirements of this specification. This feature set is
appropriate for use in software certification, interoperability
testing, and implementation reports. For each feature, this section
provides the following information:
o A human-readable name
o An informational description
o A reference to the particular section of this document that
normatively defines the feature
o Whether the feature applies to the Client role, the Server role,
or both (where "N/A" signifies that the feature is not applicable
to the specified role)
o Whether the feature MUST or SHOULD be implemented, where the
capitalized terms are to be understood as described in [TERMS]
Note: The feature set specified here attempts to adhere to the
concepts and formats proposed by Larry Masinter within the IETF's
NEWTRK Working Group in 2005, as captured in [INTEROP]. Although
this feature set is more detailed than called for by [REPORTS], it
provides a suitable basis for the generation of implementation
reports to be submitted in support of advancing this specification
from Proposed Standard to Draft Standard in accordance with
[PROCESS].
Feature: address-domain-length
Description: Ensure that the domainpart of an XMPP address is
limited to 1023 bytes in length.
Section: Section 3.2
Roles: Both MUST.
Feature: address-domain-prep
Description: Ensure that the domainpart of an XMPP address conforms
to the Nameprep profile of Stringprep.
Section: Section 3.2
Roles: Client SHOULD, Server MUST.
Feature: address-localpart-length
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Protocol (XMPP): Instant Messaging and Presence",
draft-ietf-xmpp-3921bis-05 (work in progress), March 2010.
[XMPP-URI]
Saint-Andre, P., "Internationalized Resource Identifiers
(IRIs) and Uniform Resource Identifiers (URIs) for the
Extensible Messaging and Presence Protocol (XMPP)",
RFC 5122, February 2008.
Appendix A. NodeprepA.1. Introduction
This appendix defines the "Nodeprep" profile of stringprep. As such,
it specifies processing rules that will enable users to enter
internationalized localparts in the Extensible Messaging and Presence
Protocol (XMPP) and have the highest chance of getting the content of
the strings correct. (An XMPP localpart is the optional portion of
an XMPP address that precedes an XMPP domainpart and the '@'
separator; it is often but not exclusively associated with an instant
messaging username.) These processing rules are intended only for
XMPP localparts and are not intended for arbitrary text or any other
aspect of an XMPP address.
This profile defines the following, as required by [STRINGPREP]:
o The intended applicability of the profile: internationalized
localparts within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in Section 2 of this Appendix
o The mappings used: specified in Section 3
o The Unicode normalization used: specified in Section 4
o The characters that are prohibited as output: specified in Section5
o Bidirectional character handling: specified in Section 6A.2. Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of [STRINGPREP].
A.3. Mapping
This profile specifies mapping using the following tables from
[STRINGPREP]:
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o The Unicode normalization used: specified in Section 4
o The characters that are prohibited as output: specified in Section5
o Bidirectional character handling: specified in Section 6B.2. Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of [STRINGPREP].
B.3. Mapping
This profile specifies mapping using the following tables from
[STRINGPREP]:
Table B.1
B.4. Normalization
This profile specifies the use of Unicode normalization form KC, as
described in [STRINGPREP].
B.5. Prohibited Output
This profile specifies the prohibition of using the following tables
from [STRINGPREP].
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9
B.6. Bidirectional Characters
This profile specifies checking bidirectional strings, as described
in Section 6 of [STRINGPREP].
Appendix C. XML Schemas
Because validation of XML streams and stanzas is optional, the
following XML schemas are provided for descriptive purposes only.
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<xs:element ref='remote-server-timeout'/>
<xs:element ref='resource-constraint'/>
<xs:element ref='service-unavailable'/>
<xs:element ref='subscription-required'/>
<xs:element ref='undefined-condition'/>
<xs:element ref='unexpected-request'/>
<xs:element ref='unknown-sender'/>
</xs:choice>
</xs:group>
<xs:element name='text'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:string'>
<xs:attribute ref='xml:lang' use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
Appendix D. Contact Addresses
Consistent with [MAILBOXES], an organization that offers an XMPP
service SHOULD provide an Internet mailbox of "XMPP" for inquiries
related to that service, where the host portion of the resulting
mailto URI MUST be the organization's domain, not the domain of the
XMPP service itself (e.g., the XMPP service might be offered at
im.example.com but the Internet mailbox would be <xmpp@example.com>).
Appendix E. Account Provisioning
Account provisioning is out of scope for this specification.
Possible methods for account provisioning include account creation by
a server administrator and in-band account registration using the
'jabber:iq:register' namespace as documented in [XEP-0077].
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Based on consensus derived from implementation and deployment
experience as well as formal interoperability testing, the following
substantive modifications were made from RFC 3920.
o Corrected the ABNF syntax for JIDs to prevent zero-length
localparts, domainparts, and resourceparts.
o To avoid confusion with the term "node" as used in [XEP-0030] and
[XEP-0060] (see also [XEP-0271]), changed the term "node
identifier" to "localpart" (but retained the name "Nodeprep" for
backward compatibility).
o To avoid confusion with the terms "resource" and "identifier" as
used in [URI], changed the term "resource identifier" to
"resourcepart".
o Corrected the nameprep processing rules to require use of the
UseSTD3ASCIIRules flag.
o Recommended or mandated use of the 'from' and 'to' attributes on
stream headers.
o More fully specified stream closing handshake.
o Specified recommended stream reconnection algorithm.
o Specified return of <restricted-xml/> stream error in response to
receipt of prohibited XML features.
o Specified that the SASL SCRAM mechanism is a mandatory-to-
implement technology.
o Specified that TLS plus the SASL PLAIN mechanism is a mandatory-
to-implement technology for client-to-server connections.
o Specified that support for the SASL EXTERNAL mechanism is required
for servers but only recommended for clients (since end-user X.509
certificates are difficult to obtain and not yet widely deployed).
o More clearly specified the certificate profile for both public key
certificates and issuer certificates.
o Added the <reset/> streams error condition to handle expired/
revoked certificates or the addition of security-critical features
to an existing stream.
o Added the <account-disabled/>, <credentials-expired/>,
<encryption-required/>, <malformed-request/>, and <transition-
needed/> SASL error conditions to handle error flows mistakenly
left out of RFC 3920 or discussed in RFC 4422 but not in RFC 2222.
o Added the <not-modified/> stanza error condition to enable
potential ETags usage.
o Removed unnecessary requirement for escaping of characters that
map to certain predefined entities, which do not need to be
escaped in XML.
o Clarified the process of DNS SRV lookups and fallbacks.
o Clarified the handling of SASL security layers.
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o Clarified the handling of stream features, regularized use of the
<required/> child element, and defined use of the <optional/>
child element.
o Clarified the handling of data that violates the well-formedness
definitions for XML 1.0 and XML namespaces.
o Specified the security considerations in more detail, especially
with regard to presence leaks and denial of service attacks.
o Moved historical documentation of the server dialback protocol
from this specification to a separate specification maintained by
the XMPP Standards Foundation.
In addition, numerous changes of an editorial nature were made in
order to more fully specify and clearly explain XMPP.
Appendix G. Copying Conditions
Regarding this entire document or any portion of it, the author makes
no guarantees and is not responsible for any damage resulting from
its use. The author grants irrevocable permission to anyone to use,
modify, and distribute it in any way that does not diminish the
rights of anyone else to use, modify, and distribute it, provided
that redistributed derivative works do not contain misleading author
or version information. Derivative works need not be licensed under
similar terms.
Index
B
Bare JID 19
C
Connected Resource 82
D
Domain Identifier 17
E
Entity 16
Error Stanza 94
Extended Content 110
F
Full JID 19
I
Initial Stream 24
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